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 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      units - unit conversion and calculation program

      'units' [options] [from-unit [to-unit]]

      The 'units' program converts quantities expressed in various systems
      of measurement to their equivalents in other systems of measurement.
      Like many similar programs, it can handle multiplicative scale
      changes. It can also handle nonlinear conversions such as Fahrenheit
      to Celsius; see Temperature Conversions.  The program can also perform
      conversions from and to sums of units, such as converting between
      meters and feet plus inches.

      Basic operation is simple: you enter the units that you want to con-
      vert from and the units that you want to convert to.  You can use the
      program interactively with prompts, or you can use it from the command

      Beyond simple unit conversions, 'units' can be used as a general-
      purpose scientific calculator that keeps track of units in its calcu-
      lations.  You can form arbitrary complex mathematical expressions of
      dimensions including sums, products, quotients, powers, and even roots
      of dimensions.  Thus you can ensure accuracy and dimensional con-
      sistency when working with long expressions that involve many dif-
      ferent units that may combine in complex ways; for an illustration,
      see Complicated Unit Expressions.

      The units are defined in an external data file.  You can use the
      extensive data file that comes with this program, or you can provide
      your own data file to suit your needs.  You can also use your own data
      file to supplement the standard data file.

      You can change the default behavior of 'units' with various options
      given on the command line. See Invoking Units for a description of the
      available options.

      To invoke units for interactive use, type 'units' at your shell
      prompt.  The program will print something like this:

         Currency exchange rates from on 2014-03-05
         2860 units, 109 prefixes, 85 nonlinear units

         You have:

      At the 'You have:' prompt, type the quantity and units that you are
      converting from.  For example, if you want to convert ten meters to
      feet, type '10 meters'.  Next, 'units' will print 'You want:'.  You
      should type the units you want to convert to.  To convert to feet, you

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 UNITS(1)                                                           UNITS(1)
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      would type 'feet'.  If the 'readline' library was compiled in then tab
      will complete unit names. See Readline Support for more information
      about 'readline'.  To quit the program under Unix, press Ctrl-C or
      Ctrl-D. Under Windows, press Ctrl-C or Ctrl-Z; with the latter, you
      may also need to press Enter.

      The result will be displayed in two ways.  The first line of output,
      which is marked with a '*' to indicate multiplication, gives the
      result of the conversion you have asked for.  The second line of out-
      put, which is marked with a '/' to indicate division, gives the
      inverse of the conversion factor.  If you convert 10 meters to feet,
      'units' will print

             * 32.808399
             / 0.03048

      which tells you that 10 meters equals about 32.8 feet.  The second
      number gives the conversion in the opposite direction.  In this case,
      it tells you that 1 foot is equal to about 0.03 dekameters since the
      dekameter is 10 meters.  It also tells you that 1/32.8 is about 0.03.

      The 'units' program prints the inverse because sometimes it is a more
      convenient number.  In the example above, for example, the inverse
      value is an exact conversion: a foot is exactly 0.03048 dekameters.
      But the number given the other direction is inexact.

      If you convert grains to pounds, you will see the following:

         You have: grains
         You want: pounds
                 * 0.00014285714
                 / 7000

         From the second line of the output you can immediately see that a
      grain is equal to a seven thousandth of a pound.  This is not so obvi-
      ous from the first line of the output.  If you find  the output format
      confusing, try using the '--verbose' option:

         You have: grain
         You want: aeginamina
                 grain = 0.00010416667 aeginamina
                 grain = (1 / 9600) aeginamina

      If you request a conversion between units that measure reciprocal
      dimensions, then 'units' will display the conversion results with an
      extra note indicating that reciprocal conversion has been done:

         You have: 6 ohms
         You want: siemens
                 reciprocal conversion
                 * 0.16666667

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 UNITS(1)                                                           UNITS(1)
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                 / 6

      Reciprocal conversion can be suppressed by using the '--strict'
      option.  As usual, use the '--verbose' option to get more comprehensi-
      ble output:

         You have: tex
         You want: typp
                 reciprocal conversion
                 1 / tex = 496.05465 typp
                 1 / tex = (1 / 0.0020159069) typp

         You have: 20 mph
         You want: sec/mile
                 reciprocal conversion
                 1 / 20 mph = 180 sec/mile
                 1 / 20 mph = (1 / 0.0055555556) sec/mile

      If you enter incompatible unit types, the 'units' program will print a
      message indicating that the units are not conformable and it will
      display the reduced form for each unit:

         You have: ergs/hour
         You want: fathoms kg^2 / day
         conformability error
                 2.7777778e-11 kg m^2 / sec^3
                 2.1166667e-05 kg^2 m / sec

      If you only want to find the reduced form or definition of a unit,
      simply press Enter at the 'You want:' prompt.  Here is an example:

         You have: jansky
         You want:
                 Definition: fluxunit = 1e-26 W/m^2 Hz = 1e-26 kg / s^2

      The output from 'units' indicates that the jansky is defined to be
      equal to a fluxunit which in turn is defined to be a certain combina-
      tion of watts, meters, and hertz.  The fully reduced (and in this case
      somewhat more cryptic) form appears on the far right.

      Some named units are treated as dimensionless in some situations.
      These units include the radian and steradian.  These units will be
      treated as equal to 1 in units conversions.  Power is equal to torque
      times angular velocity.  This conversion can only be performed if the
      radian is dimensionless.

         You have: (14 ft lbf) (12 radians/sec)
         You want: watts
                 * 227.77742
                 / 0.0043902509

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 UNITS(1)                                                           UNITS(1)
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      It is also possible to compute roots and other non-integer powers of
      dimensionless units; this allows computations such as the altitude of
      geosynchronous orbit:

         You have: cuberoot(G earthmass / (circle/siderealday)^2) - earthradius
         You want: miles
                 * 22243.267
                 / 4.4957425e-05

      Named dimensionless units are not treated as dimensionless in other
      contexts.  They cannot be used as exponents so for example,
      'meter^radian' is forbidden.

      If you want a list of options you can type '?' at the 'You want:'
      prompt.  The program will display a list of named units that are con-
      formable with the unit that you entered at the 'You have:' prompt
      above.  Conformable unit combinations will not appear on this list.

      Typing 'help' at either prompt displays a short help message.  You can
      also type 'help' followed by a unit name.  This will invoke a pager on
      the units data base at the point where that unit is defined.  You can
      read the definition and comments that may give more details or histor-
      ical information about the unit.  (You can generally quit out of the
      page by pressing 'q'.)

      Typing 'search' text will display a list of all of the units whose
      names contain text as a substring along with their definitions.  This
      may help in the case where you aren't sure of the right unit name.

      The 'units' program can perform units conversions non-interactively
      from the command line.  To do this, type the command, type the origi-
      nal unit expression, and type the new units you want.  If a units
      expression contains non-alphanumeric characters, you may need to pro-
      tect it from interpretation by the shell using single or double quote

      If you type

         units "2 liters" quarts

      then 'units' will print

             * 2.1133764
             / 0.47317647

      and then exit.  The output tells you that 2 liters is about 2.1
      quarts, or alternatively that a quart is about 0.47 times 2 liters.

      If the conversion is successful, then 'units' will return success
      (zero) to the calling environment.  If you enter  non-conformable

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 UNITS(1)                                                           UNITS(1)
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      units then 'units' will print a message giving the reduced form of
      each unit and it will return failure (nonzero) to the calling environ-

      When you invoke 'units' with only one argument, it will print out the
      definition of the specified unit.  It will return failure if the unit
      is not defined and success if the unit is defined.

      The conversion information is read from a units data file that is
      called 'definitions.units' and is usually located in the
      '/usr/share/units' directory.  If you invoke 'units' with the '-V'
      option, it will print the location of this file.  The default file
      includes definitions for all familiar units, abbreviations and metric
      prefixes.  It also includes many obscure or archaic units.  Many com-
      mon spelled-out numbers (e.g., 'seventeen') are recognized.

      Many constants of nature are defined, including these:

         pi          ratio of circumference to diameter
         c           speed of light
         e           charge on an electron
         force       acceleration of gravity
         mole        Avogadro's number
         water       pressure per unit height of water
         Hg          pressure per unit height of mercury
         au          astronomical unit
         k           Boltzman's constant
         mu0         permeability of vacuum
         epsilon0    permittivity of vacuum
         G           Gravitational constant
         mach        speed of sound

      The standard data file includes atomic masses for all of the elements
      and numerous other constants.  Also included are the densities of
      various ingredients used in baking so that '2 cups flour_sifted' can
      be converted to 'grams'.  This is not an exhaustive list.  Consult the
      units data file to see the complete list, or to see the definitions
      that are used.

      The 'pound' is a unit of mass.  To get force, multiply by the force
      conversion unit 'force' or use the shorthand 'lbf'.  (Note that 'g' is
      already taken as the standard abbreviation for the gram.)  The unit
      'ounce' is also a unit of mass.  The fluid ounce is 'fluidounce' or
      'floz'.  When British capacity units differ from their US counter-
      parts, such as the British Imperial gallon, the unit is defined both
      ways with 'br' and 'us' prefixes.  Your locale settings will determine
      the value of the unprefixed unit.  Currency is prefixed with its coun-
      try name: 'belgiumfranc', 'britainpound'.

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 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      When searching for a unit, if the specified string does not appear
      exactly as a unit name, then the 'units' program will try to remove a
      trailing 's', 'es'.  Next units will replace a trailing 'ies' with
      'y'.  If that fails, 'units' will check for a prefix.  The database
      includes all of the standard metric prefixes.  Only one prefix is per-
      mitted per unit, so 'micromicrofarad' will fail.  However, prefixes
      can appear alone with no unit following them, so 'micro*microfarad'
      will work, as will 'micro microfarad'.

      To find out which units and prefixes are available, read the standard
      units data file, which is extensively annotated.

    English Customary Units
      English customary units differ in various ways in different regions.
      In Britain a complex system of volume measurements featured different
      gallons for different materials such as a wine gallon and ale gallon
      that different by twenty percent.  This complexity was swept away in
      1824 by a reform that created an entirely new gallon, the British
      Imperial gallon defined as the volume occupied by ten pounds of water.
      Meanwhile in the USA the gallon is derived from the 1707 Winchester
      wine gallon, which is 231 cubic inches.  These gallons differ by about
      twenty percent.  By default if 'units' runs in the 'en_GB' locale you
      will get the British volume measures.  If it runs in the 'en_US'
      locale you will get the US volume measures.  In other locales the
      default values are the US definitions.  If you wish to force different
      definitions then set the environment variable 'UNITS_ENGLISH' to
      either 'US' or 'GB' to set the desired definitions independent of the

      Before 1959, the value of a yard (and other units of measure defined
      in terms of it) differed slightly among English-speaking countries.
      In 1959, Australia, Canada, New Zealand, the United Kingdom, the
      United States, and South Africa adopted the Canadian value of 1 yard =
      0.9144 m (exactly), which was approximately halfway between the values
      used by the UK and the US; it had the additional advantage of making
      1 inch = 2.54 cm (exactly).  This new standard was termed the Interna-
      tional Yard.  Australia, Canada, and the UK then defined all customary
      lengths in terms of the International Yard (Australia did not define
      the furlong or rod); because many US land surveys were in terms of the
      pre-1959 units, the US continued to define customary surveyors' units
      (furlong, chain, rod, and link) in terms of the previous value for the
      foot, which was termed the US survey foot.  The US defined a US survey
      mile as 5280 US survey feet, and defined a statute mile as a US survey
      mile.  The US values for these units differ from the international
      values by about 2 ppm.

      The 'units' program uses the international values for these units; the
      US values can be obtained by using either the 'US' or the 'survey'
      prefix.  In either case, the simple familiar relationships among the
      units are maintained, e.g., 1 'furlong' = 660 'ft', and 1 'USfurlong'
      = 660 'USft', though the metric equivalents differ slightly between

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 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      the two cases.  The 'US' prefix or the 'survey' prefix can also be
      used to obtain the US survey mile and the value of the US yard prior
      to 1959, e.g., 'USmile' or 'surveymile' (but not 'USsurveymile').  To
      get the US value of the statute mile, use either 'USstatutemile' or

      Except for distances that extend over hundreds of miles (such as in
      the US State Plane Coordinate System), the differences in the miles
      are usually insignificant:

         You have: 100 surveymile - 100 mile
         You want: inch
                 * 12.672025
                 / 0.078913984

      The pre-1959 UK values for these units can be obtained with the prefix

      In the US, the acre is officially defined in terms of the US survey
      foot, but 'units' uses a definition based on the international foot.
      If you want the official US acre use 'USacre' and similarly use
      'USacrefoot' for the official US version of that unit.  The difference
      between these units is about 4 parts per million.

      You can enter more complicated units by combining units with opera-
      tions such as multiplication, division, powers, addition, subtraction,
      and parentheses for grouping.  You can use the customary symbols for
      these operators when 'units' is invoked with its default options.
      Additionally, 'units' supports some extensions, including high prior-
      ity multiplication using a space, and a high priority numerical divi-
      sion operator ('|') that can simplify some expressions.

      You multiply units using a space or an asterisk ('*').  The next exam-
      ple shows both forms:

         You have: arabicfoot * arabictradepound * force
         You want: ft lbf
                 * 0.7296
                 / 1.370614

      You can divide units using the slash ('/') or with 'per':

         You have: furlongs per fortnight
         You want: m/s
                 * 0.00016630986
                 / 6012.8727

      You can use parentheses for grouping:

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 UNITS(1)                                                           UNITS(1)
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         You have: (1/2) kg / (kg/meter)
         You want: league
                 * 0.00010356166
                 / 9656.0833

      White space surrounding operators is optional, so the previous example
      could have used '(1/2)kg/(kg/meter)'.  As a consequence, however,
      hyphenated spelled-out numbers (e.g., 'forty-two') cannot be used;
      'forty-two' is interpreted as '40 - 2'.

      Multiplication using a space has a higher precedence than division
      using a slash and is evaluated left to right; in effect, the first '/'
      character marks the beginning of the denominator of a unit expression.
      This makes it simple to enter a quotient with several terms in the
      denominator: 'J / mol K'.  The '*' and '/' operators have the same
      precedence, and are evaluated left to right; if you multiply with '*',
      you must group the terms in the denominator with parentheses:
      'J / (mol * K)'.

      The higher precedence of the space operator may not always be advanta-
      geous.  For example, 'm/s s/day' is equivalent to 'm / s s day' and
      has dimensions of length per time cubed.  Similarly, '1/2 meter'
      refers to a unit of reciprocal length equivalent to 0.5/meter, perhaps
      not what you would intend if you entered that expression.  The get a
      half meter you would need to use parentheses: '(1/2) meter'.  The '*'
      operator is convenient for multiplying a sequence of quotients.  For
      example, 'm/s * s/day' is equivalent to 'm/day'.  Similarly, you could
      write '1/2 * meter' to get half a meter.

      The 'units' program supports another option for numerical fractions:
      you can indicate division of numbers with the vertical bar ('|'), so
      if you wanted half a meter you could write '1|2 meter'.  You cannot
      use the vertical bar to indicate division of non-numerical units
      (e.g., 'm|s' results in an error message).

      Powers of units can be specified using the '^' character, as shown in
      the following example, or by simple concatenation of a unit and its
      exponent: 'cm3' is equivalent to 'cm^3'; if the exponent is more than
      one digit, the '^' is required.  You can also use '**' as an exponent

         You have: cm^3
         You want: gallons
                 * 0.00026417205
                 / 3785.4118

      Concatenation only works with a single unit name: if you write
      '(m/s)2', 'units' will treat it as multiplication by 2.  When a unit
      includes a prefix, exponent operators apply to the combination, so
      'centimeter3' gives cubic centimeters.  If you separate the prefix
      from the unit with any multiplication operator (e.g., 'centi

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      meter^3'), the prefix is treated as a separate unit, so the exponent
      applies only to the unit without the prefix.  The second example is
      equivalent to 'centi * (meter^3)', and gives a hundredth of a cubic
      meter, not a cubic centimeter.  The 'units' program is limited inter-
      nally to products of 99 units; accordingly, expressions like
      'meter^100' or 'joule^34' (represented internally as
      'kg^34 m^68 / s^68') will fail.

      The '|' operator has the highest precedence, so you can write the
      square root of two thirds as '2|3^1|2'.  The '^' operator has the
      second highest precedence, and is evaluated right to left, as usual:

         You have: 5 * 2^3^2
         You want:
                 Definition: 2560

      With a dimensionless base unit, any dimensionless exponent is meaning-
      ful (e.g., 'pi^exp(2.371)').  Even though angle is sometimes treated
      as dimensionless, exponents cannot have dimensions of angle:

         You have: 2^radian
         Exponent not dimensionless

      If the base unit is not dimensionless, the exponent must be a rational
      number p/q, and the dimension of the unit must be a power of q, so
      'gallon^2|3' works but 'acre^2|3' fails.  An exponent using the slash
      ('/') operator (e.g., 'gallon^(2/3)') is also acceptable; the
      parentheses are needed because the precedence of '^' is higher than
      that of '/'.  Since 'units' cannot represent dimensions with exponents
      greater than 99, a fully reduced exponent must have q < 100.  When
      raising a non-dimensionless unit to a power, 'units' attempts to con-
      vert a decimal exponent to a rational number with q < 100.  If this is
      not possible 'units' displays an error message:

         You have: ft^1.234
         Base unit not dimensionless; rational exponent required

      A decimal exponent must match its rational representation to machine
      precision, so 'acre^1.5' works but 'gallon^0.666' does not.

    Sums and Differences of Units
      You may sometimes want to add values of different units that are out-
      side the SI.  You may also wish to use 'units' as a calculator that
      keeps track of units.  Sums of conformable units are written with the
      '+' character, and differences with the '-' character.

         You have: 2 hours + 23 minutes + 32 seconds
         You want: seconds
                 * 8612
                 / 0.00011611705

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         You have: 12 ft + 3 in
         You want: cm
                 * 373.38
                 / 0.0026782366

         You have: 2 btu + 450 ft lbf
         You want: btu
                 * 2.5782804
                 / 0.38785542

      The expressions that are added or subtracted must reduce to identical
      expressions in primitive units, or an error message will be displayed:

         You have: 12 printerspoint - 4 heredium
         Illegal sum of non-conformable units

      As usual, the precedence for '+' and '-' is lower than that of the
      other operators.  A fractional quantity such as 2 1/2 cups can be
      given as '(2+1|2) cups'; the parentheses are necessary because multi-
      plication has higher precedence than addition.  If you omit the
      parentheses, 'units' attempts to add '2' and '1|2 cups', and you get
      an error message:

         You have: 2+1|2 cups
         Illegal sum or difference of non-conformable units

      The expression could also be correctly written as '(2+1/2) cups'.  If
      you write '2 1|2 cups' the space is interpreted as multiplication so
      the result is the same as '1 cup'.

      The '+' and '-' characters sometimes appears in exponents like
      '3.43e+8'.  This leads to an ambiguity in an expression like '3e+2
      yC'.  The unit 'e' is a small unit of charge, so this can be regarded
      as equivalent to '(3e+2) yC' or '(3 e)+(2 yC)'.  This ambiguity is
      resolved by always interpreting '+' and '-' as part of an exponent if

    Numbers as Units
      For 'units', numbers are just another kind of unit.  They can appear
      as many times as you like and in any order in a unit expression.  For
      example, to find the volume of a box that is 2 ft by 3 ft by 12 ft in
      steres, you could do the following:

         You have: 2 ft 3 ft 12 ft
         You want: stere
                 * 2.038813
                 / 0.49048148

         You have: $ 5 / yard

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                                7 March 2017

         You want: cents / inch
                 * 13.888889
                 / 0.072

      And the second example shows how the dollar sign in the units conver-
      sion can precede the five.  Be careful: 'units' will interpret '$5'
      with no space as equivalent to 'dollar^5'.

    Built-in Functions
      Several built-in functions are provided: 'sin', 'cos', 'tan', 'ln',
      'log', 'log2', 'exp', 'acos', 'atan' and 'asin'.  The 'sin', 'cos',
      and 'tan' functions require either a dimensionless argument or an
      argument with dimensions of angle.

         You have: sin(30 degrees)
         You want:
                 Definition: 0.5

         You have: sin(pi/2)
         You want:
                 Definition: 1

         You have: sin(3 kg)
         Unit not dimensionless

      The other functions on the list require dimensionless arguments.  The
      inverse trigonometric functions return arguments with dimensions of

      If you wish to take roots of units, you may use the 'sqrt' or
      'cuberoot' functions.  These functions require that the argument have
      the appropriate root.  You can obtain higher roots by using fractional

         You have: sqrt(acre)
         You want: feet
                 * 208.71074
                 / 0.0047913202

         You have: (400 W/m^2 / stefanboltzmann)^(1/4)
         You have:
                 Definition: 289.80882 K

         You have: cuberoot(hectare)
         Unit not a root

    Previous Result
      You can insert the result of the previous conversion using the under-
      score ('_').  It is useful when you want to convert the same input to

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                                7 March 2017

      several different units, for example

         You have: 2.3 tonrefrigeration
         You want: btu/hr
                 * 27600
                 / 3.6231884e-005
         You have: _
         You want: kW
                 * 8.0887615
                 / 0.12362832

      Suppose you want to do some deep frying that requires an oil depth of
      2 inches.  You have 1/2 gallon of oil, and want to know the largest-
      diameter pan that will maintain the required depth.  The nonlinear
      unit 'circlearea' gives the radius of the circle (see Other Nonlinear
      Units, for a more detailed description) in SI units; you want the
      diameter in inches:

         You have: 1|2 gallon / 2 in
         You want: circlearea
                 0.10890173 m
         You have: 2 _
         You want: in
                 * 8.5749393
                 / 0.1166189

      In most cases, surrounding white space is optional, so the previous
      example could have used '2_'.  If '_' follows a non-numerical unit
      symbol, however, the space is required:

         You have: m_
         Parse error

      When '_' is followed by a digit, the operation is multiplication
      rather than exponentiation, so that '_2', is equivalent to '_ * 2'
      rather than '_^2'.

      You can use the '_' symbol any number of times; for example,

         You have: m
         You want:
                 Definition: 1 m
         You have: _ _
         You want:
                 Definition: 1 m^2

      Using '_' before a conversion has been performed (e.g., immediately
      after invocation) generates an error:

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                                7 March 2017

         You have: _
         No previous result; '_' not set

      Accordingly, '_' serves no purpose when 'units' is invoked non-

      If 'units' is invoked with the '--verbose' option (see Invoking
      Units), the value of '_' is not expanded:

         You have: mile
         You want: ft
                 mile = 5280 ft
                 mile = (1 / 0.00018939394) ft
         You have: _
         You want: m
                 _ = 1609.344 m
                 _ = (1 / 0.00062137119) m

      You can give '_' at the 'You want:' prompt, but it usually is not very

    Complicated Unit Expressions
      The 'units' program is especially helpful in ensuring accuracy and
      dimensional consistency when converting lengthy unit expressions.  For
      example, one form of the Darcy-Weisbach fluid-flow equation is

           Delta P = (8 / pi)^2 (rho fLQ^2) / d^5,

      where Delta P is the pressure drop, rho is the mass density, f is the
      (dimensionless) friction factor, L is the length of the pipe, Q is the
      volumetric flow rate, and d is the pipe diameter.  It might be desired
      to have the equation in the form

           Delta P = A1 rho fLQ^2 / d^5

      that accepted the user's normal units; for typical units used in the
      US, the required conversion could be something like

         You have: (8/pi^2)(lbm/ft^3)ft(ft^3/s)^2(1/in^5)
         You want: psi
                 * 43.533969
                 / 0.022970568

      The parentheses allow individual terms in the expression to be entered
      naturally, as they might be read from the formula.  Alternatively, the
      multiplication could be done with the '*' rather than a space; then
      parentheses are needed only around 'ft^3/s' because of its exponent:

         You have: 8/pi^2 * lbm/ft^3 * ft * (ft^3/s)^2 /in^5
         You want: psi

                                   - 13 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

                 * 43.533969
                 / 0.022970568

      Without parentheses, and using spaces for multiplication, the previous
      conversion would need to be entered as

         You have: 8 lb ft ft^3 ft^3 / pi^2 ft^3 s^2 in^5
         You want: psi
                 * 43.533969
                 / 0.022970568

    Backwards Compatibility:
      '*' and '-' The original 'units' assigned multiplication a higher pre-
      cedence than division using the slash.  This differs from the usual
      precedence rules, which give multiplication and division equal pre-
      cedence, and can be confusing for people who think of units as a cal-

      The star operator ('*') included in this 'units' program has, by
      default, the same precedence as division, and hence follows the usual
      precedence rules.  For backwards compatibility you can invoke 'units'
      with the '--oldstar' option.  Then '*' has a higher precedence than
      division, and the same precedence as multiplication using the space.

      Historically, the hyphen ('-') has been used in technical publications
      to indicate products of units, and the original 'units' program
      treated it as a multiplication operator.  Because 'units' provides
      several other ways to obtain unit products, and because '-' is a sub-
      traction operator in general algebraic expressions, 'units' treats the
      binary '-' as a subtraction operator by default.  For backwards compa-
      tibility use the '--product' option, which causes 'units' to treat the
      binary '-' operator as a product operator.  When '-' is a multiplica-
      tion operator it has the same precedence as multiplication with a
      space, giving it a higher precedence than division.

      When '-' is used as a unary operator it negates its operand.  Regard-
      less of the 'units' options, if '-' appears after '(' or after '+'
      then it will act as a negation operator.  So you can always compute 20
      degrees minus 12 minutes by entering '20 degrees + -12 arcmin'.  You
      must use this construction when you define new units because you can-
      not know what options will be in force when your definition is pro-

      Nonlinear units are represented using functional notation.  They make
      possible nonlinear unit conversions such as temperature.

    Temperature Conversions
      Conversions between temperatures are different from linear conversions
      between temperature increments-see the example below.  The absolute
      temperature conversions are handled by units starting with 'temp', and

                                   - 14 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      you must use functional notation.  The temperature-increment conver-
      sions are done using units starting with 'deg' and they do not require
      functional notation.

         You have: tempF(45)
         You want: tempC

         You have: 45 degF
         You want: degC
                 * 25
                 / 0.04

      Think of 'tempF(x)' not as a function but as a notation that indicates
      that x should have units of 'tempF' attached to it.  See Defining Non-
      linear Units.  The first conversion shows that if it's 45 degrees
      Fahrenheit outside, it's 7.2 degrees Celsius.  The second conversion
      indicates that a change of 45 degrees Fahrenheit corresponds to a
      change of 25 degrees Celsius.  The conversion from 'tempF(x)' is to
      absolute temperature, so that

         You have: tempF(45)
         You want: degR
                 * 504.67
                 / 0.0019814929

      gives the same result as

         You have: tempF(45)
         You want: tempR
                 * 504.67
                 / 0.0019814929

      But if you convert 'tempF(x)' to 'degC', the output is probably not
      what you expect:

         You have: tempF(45)
         You want: degC
                 * 280.37222
                 / 0.0035666871

      The result is the temperature in K, because 'degC' is defined as 'K',
      the Kelvin. For consistent results, use the 'tempX' units when con-
      verting to a temperature rather than converting a temperature incre-

      The 'tempC()' and 'tempF()' definitions are limited to positive abso-
      lute temperatures, and giving a value that would result in a negative
      absolute temperature generates an error message:

                                   - 15 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

         You have: tempC(-275)
         Argument of function outside domain

    Other Nonlinear Units
      Some other examples of nonlinear units are numerous different ring
      sizes and wire gauges, the grit sizes used for abrasives, the decibel
      scale, shoe size, scales for the density of sugar (e.g., baume).  The
      standard data file also supplies units for computing the area of a
      circle and the volume of a sphere.  See the standard units data file
      for more details.  Wire gauges with multiple zeroes are signified
      using negative numbers where two zeroes is '-1'.  Alternatively, you
      can use the synonyms 'g00', 'g000', and so on that are defined in the
      standard units data file.

         You have: wiregauge(11)
         You want: inches
                 * 0.090742002
                 / 11.020255

         You have: brwiregauge(g00)
         You want: inches
                 * 0.348
                 / 2.8735632

         You have: 1 mm
         You want: wiregauge

         You have: grit_P(600)
         You want: grit_ansicoated

      The last example shows the conversion from P graded sand paper, which
      is the European standard and may be marked ``P600'' on the back, to
      the USA standard.

      You can compute the area of a circle using the nonlinear unit,
      'circlearea'.  You can also do this using the circularinch or cir-
      cleinch.  The next example shows two ways to compute the area of a
      circle with a five inch radius and one way to compute the volume of a
      sphere with a radius of one meter.

         You have: circlearea(5 in)
         You want: in2
                 * 78.539816
                 / 0.012732395

         You have: 10^2 circleinch
         You want: in2

                                   - 16 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

                 * 78.539816
                 / 0.012732395

         You have: spherevol(meter)
         You want: ft3
                 * 147.92573
                 / 0.0067601492

      The inverse of a nonlinear conversion is indicated by prefixing a
      tilde ('~') to the nonlinear unit name:

         You have: ~wiregauge(0.090742002 inches)
         You want:
                 Definition: 11

      You can give a nonlinear unit definition without an argument or
      parentheses, and press Enter at the 'You want:' prompt to get the
      definition of a nonlinear unit; if the definition is not valid for all
      real numbers, the range of validity is also given.  If the definition
      requires specific units this information is also displayed:

         You have: tempC
                 Definition: tempC(x) = x K + stdtemp
                             defined for x >= -273.15
         You have: ~tempC
                 Definition: ~tempC(tempC) = (tempC +(-stdtemp))/K
                             defined for tempC >= 0 K
         You have: circlearea
                 Definition: circlearea(r) = pi r^2
                             r has units m

      To see the definition of the inverse use the '~' notation.  In this
      case the parameter in the functional definition will usually be the
      name of the unit.  Note that the inverse for 'tempC' shows that it
      requires units of 'K' in the specification of the allowed range of
      values. Nonlinear unit conversions are described in more detail in
      Defining Nonlinear Units.

      Outside of the SI, it is sometimes desirable to convert a single unit
      to a sum of units-for example, feet to feet plus inches.  The conver-
      sion from sums of units was described in Sums and Differences of
      Units, and is a simple matter of adding the units with the '+' sign:

         You have: 12 ft + 3 in + 3|8 in
         You want: ft
                 * 12.28125
                 / 0.081424936

      Although you can similarly write a sum of units to convert to, the
      result will not be the conversion to the units in the sum, but rather

                                   - 17 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      the conversion to the particular sum that you have entered:

         You have: 12.28125 ft
         You want: ft + in + 1|8 in
                 * 11.228571
                 / 0.089058524

      The unit expression given at the 'You want:' prompt is equivalent to
      asking for conversion to multiples of '1 ft + 1 in + 1|8 in', which is
      1.09375 ft, so the conversion in the previous example is equivalent to

         You have: 12.28125 ft
         You want: 1.09375 ft
                 * 11.228571
                 / 0.089058524

      In converting to a sum of units like miles, feet and inches, you typi-
      cally want the largest integral value for the first unit, followed by
      the largest integral value for the next, and the remainder converted
      to the last unit.  You can do this conversion easily with 'units'
      using a special syntax for lists of units.  You must list the desired
      units in order from largest to smallest, separated by the semicolon
      (';') character:

         You have: 12.28125 ft
         You want: ft;in;1|8 in
                 12 ft + 3 in + 3|8 in

      The conversion always gives integer coefficients on the units in the
      list, except possibly the last unit when the conversion is not exact:

         You have: 12.28126 ft
         You want: ft;in;1|8 in
                 12 ft + 3 in + 3.00096 * 1|8 in

      The order in which you list the units is important:

         You have: 3 kg
         You want: oz;lb
                 105 oz + 0.051367866 lb

         You have: 3 kg
         You want: lb;oz
                 6 lb + 9.8218858 oz

      Listing ounces before pounds produces a technically correct result,
      but not a very useful one.  You must list the units in descending
      order of size in order to get the most useful result.

      Ending a unit list with the separator ';' has the same effect as
      repeating the last unit on the list, so 'ft;in;1|8 in;' is equivalent

                                   - 18 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      to 'ft;in;1|8 in;1|8 in'.  With the example above, this gives

         You have: 12.28126 ft
         You want: ft;in;1|8 in;
                 12 ft + 3 in + 3|8 in + 0.00096 * 1|8 in

      in effect separating the integer and fractional parts of the coeffi-
      cient for the last unit.  If you instead prefer to round the last
      coefficient to an integer you can do this with the '--round' ('-r')
      option.  With the previous example, the result is

         You have: 12.28126 ft
         You want: ft;in;1|8 in
                 12 ft + 3 in + 3|8 in (rounded down to nearest 1|8 in)

      When you use the '-r' option, repeating the last unit on the list has
      no effect (e.g., 'ft;in;1|8 in;1|8 in' is equivalent to 'ft;in;1|8
      in'), and hence neither does ending a list with a ';'.  With a single
      unit and the '-r' option, a terminal ';' does have an effect: it
      causes 'units' to treat the single unit as a list and produce a
      rounded value for the single unit.  Without the extra ';', the '-r'
      option has no effect on single unit conversions.  This example shows
      the output using the '-r' option:

         You have: 12.28126 ft
         You want: in
                 * 147.37512
                 / 0.0067854058

         You have: 12.28126 ft
         You want: in;
                 147 in (rounded down to nearest in)

      Each unit that appears in the list must be conformable with the first
      unit on the list, and of course the listed units must also be conform-
      able with the unit that you enter at the 'You have:' prompt.

         You have: meter
         You want: ft;kg
         conformability error
                 ft = 0.3048 m
                 kg = 1 kg

         You have: meter
         You want: lb;oz
         conformability error
                 1 m
                 0.45359237 kg

                                   - 19 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      In the first case, 'units' reports the disagreement between units
      appearing on the list.  In the second case, 'units' reports disagree-
      ment between the unit you entered and the desired conversion.  This
      conformability error is based on the first unit on the unit list.

      Other common candidates for conversion to sums of units are angles and

         You have: 23.437754 deg
         You want; deg;arcmin;arcsec
             23 deg + 26 arcmin + 15.9144 arcsec

         You have: 7.2319 hr
         You want: hr;min;sec
             7 hr + 13 min + 54.84 sec

      In North America, recipes for cooking typically measure ingredients by
      volume, and use units that are not always convenient multiples of each
      other.  Suppose that you have a recipe for 6 and you wish to make a
      portion for 1.  If the recipe calls for 2 1/2 cups of an ingredient,
      you might wish to know the measurements in terms of measuring devices
      you have available, you could use 'units' and enter

         You have: (2+1|2) cup / 6
         You want: cup;1|2 cup;1|3 cup;1|4 cup;tbsp;tsp;1|2 tsp;1|4 tsp
                 1|3 cup + 1 tbsp + 1 tsp

      By default, if a unit in a list begins with fraction of the form 1|x
      and its multiplier is an integer, the fraction is given as the product
      of the multiplier and the numerator; for example,

         You have: 12.28125 ft
         You want: ft;in;1|8 in;
                 12 ft + 3 in + 3|8 in

      In many cases, such as the example above, this is what is wanted, but
      sometimes it is not.  For example, a cooking recipe for 6 might call
      for 5 1/4 cup of an ingredient, but you want a portion for 2, and your
      1-cup measure is not available; you might try

         You have: (5+1|4) cup / 3
         You want: 1|2 cup;1|3 cup;1|4 cup
                 3|2 cup + 1|4 cup

      This result might be fine for a baker who has a 1 1/2-cup measure (and
      recognizes the equivalence), but it may not be as useful to someone
      with more limited set of measures, who does want to do additional cal-
      culations, and only wants to know ``How many 1/2-cup measures to I
      need to add?''  After all, that's what was actually asked.  With the
      '--show-factor' option, the factor will not be combined with a unity
      numerator, so that you get

                                   - 20 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

         You have: (5+1|4) cup / 3
         You want: 1|2 cup;1|3 cup;1|4 cup
                 3 * 1|2 cup + 1|4 cup

      A user-specified fractional unit with a numerator other than 1 is
      never overridden, however-if a unit list specifies '3|4 cup;1|2 cup',
      a result equivalent to 1 1/2 cups will always be shown as '2 *
      3|4 cup' whether or not the '--show-factor' option is given.

      Some applications for unit lists may be less obvious.  Suppose that
      you have a postal scale and wish to ensure that it's accurate at 1 oz,
      but have only metric calibration weights.  You might try

         You have: 1 oz
         You want: 100 g;50 g; 20 g;10 g;5 g;2 g;1 g;
                 20 g + 5 g + 2 g + 1 g + 0.34952312 * 1 g

      You might then place one each of the 20 g, 5 g, 2 g, and 1 g weights
      on the scale and hope that it indicates close to

         You have: 20 g + 5 g + 2 g + 1 g
         You want: oz;
                 0.98767093 oz

      Appending ';' to 'oz' forces a one-line display that includes the
      unit; here the integer part of the result is zero, so it is not

      A unit list such as

         cup;1|2 cup;1|3 cup;1|4 cup;tbsp;tsp;1|2 tsp;1|4 tsp

      can be tedious to enter.  The 'units' program provides shorthand names
      for some common combinations:

         hms         hours, minutes, seconds
         dms         angle: degrees, minutes, seconds
         time        years, days, hours, minutes and seconds
         usvol       US cooking volume: cups and smaller

      Using these shorthands, or unit list aliases, you can do the following

         You have: anomalisticyear
         You want: time
                 1 year + 25 min + 3.4653216 sec
         You have: 1|6 cup
         You want: usvol
                 2 tbsp + 2 tsp

                                   - 21 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      You cannot combine a unit list alias with other units: it must appear
      alone at the 'You want:' prompt.

      You can display the definition of a unit list alias by entering it at
      the 'You have:' prompt:

         You have: dms
                 Definition: unit list, deg;arcmin;arcsec

      When you specify compact output with '--compact', '--terse' or '-t'
      and perform conversion to a unit list, 'units' lists the conversion
      factors for each unit in the list, separated by semicolons.

         You have: year
         You want: day;min;sec

      Unlike the case of regular output, zeros are included in this output

         You have: liter
         You want: cup;1|2 cup;1|4 cup;tbsp

      The '--log' option allows you to save the results of calculations in a
      file; this can be useful if you need a permanent record of your work.
      For example, the fluid-flow conversion in Complicated Unit Expres-
      sions, is lengthy, and if you were to use it in designing a piping
      system, you might want a record of it for the project file.  If the
      interactive session

         # Conversion factor A1 for pressure drop
         # dP = A1 rho f L Q^2/d^5
         You have: (8/pi^2) (lbm/ft^3)ft(ft^3/s)^2(1/in^5) # Input units
         You want: psi
                 * 43.533969
                 / 0.022970568

      were logged, the log file would contain

         ### Log started Fri Oct 02 15:55:35 2015

         # Conversion factor A1 for pressure drop
         # dP = A1 rho f L Q^2/d^5
         From: (8/pi^2) (lbm/ft^3)ft(ft^3/s)^2(1/in^5)   # Input units
         To:   psi
                 * 43.533969
                 / 0.022970568

                                   - 22 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      The time is written to the log file when the file is opened.

      The use of comments can help clarify the meaning of calculations for
      the log. The log includes conformability errors between the units at
      the 'You have:' and 'You want:' prompts, but not other errors, includ-
      ing lack of conformability of items in sums or differences or among
      items in a unit list.  For example, a conversion between zenith angle
      and elevation angle could involve

         You have: 90 deg - (5 deg + 22 min + 9 sec)
         Illegal sum or difference of non-conformable units
         You have: 90 deg - (5 deg + 22 arcmin + 9 arcsec)
         You want: dms
                 84 deg + 37 arcmin + 51 arcsec
         You have: _
         You want: deg
                 * 84.630833
                 / 0.011816024
         You have:

      The log file would contain

         From: 90 deg - (5 deg + 22 arcmin + 9 arcsec)
         To:   deg;arcmin;arcsec
                 84 deg + 37 arcmin + 51 arcsec
         From: _
         To:   deg
                 * 84.630833
                 / 0.011816024

      The initial entry error (forgetting that minutes have dimension of
      time, and that arcminutes must be used for dimensions of angle) does
      not appear in the output.  When converting to a unit list alias,
      'units' expands the alias in the log file.

      The 'From:' and 'To:' tags are written to the log file even if the '-
      -quiet' option is given.  If the log file exists when 'units' is
      invoked, the new results are appended to the log file.  The time is
      written to the log file each time the file is opened.  The '--log'
      option is ignored when 'units' is used non-interactively.

      You invoke 'units' like this:

         units [options] [from-unit [to-unit]]

      If the from-unit and to-unit are omitted, the program will use
      interactive prompts to determine which conversions to perform.  See
      Interactive Use.  If both from-unit and to-unit are given, 'units'
      will print the result of that single conversion and then exit.  If

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 UNITS(1)                                                           UNITS(1)
                                7 March 2017

      only from-unit appears on the command line, 'units' will display the
      definition of that unit and exit.  Units specified on the command line
      may need to be quoted to protect them from shell interpretation and to
      group them into two arguments.  See Command Line Use.

      The default behavior of 'units' can be changed by various options
      given on the command line.  In most cases, the options may be given in
      either short form (a single '-' followed by a single character) or
      long form ('--' followed by a word or hyphen-separated words).
      Short-form options are cryptic but require less typing; long-form
      options require more typing but are more explanatory and may be more
      mnemonic.  With long-form options you need only enter sufficient char-
      acters to uniquely identify the option to the program.  For example,
      '--out %f' works, but '--o %f' fails because 'units' has other long
      options beginning with 'o'.  However, '--q' works because '--quiet' is
      the only long option beginning with 'q'.

      Some options require arguments to specify a value (e.g., '-d 12' or
      '--digits 12').  Short-form options that do not take arguments may be
      concatenated (e.g., '-erS' is equivalent to '-e -r -S'); the last
      option in such a list may be one that takes an argument (e.g., '-
      ed 12').  With short-form options, the space between an option and its
      argument is optional (e.g., '-d12' is equivalent to '-d 12').  Long-
      form options may not be concatenated, and the space between a long-
      form option and its argument is required.  Short-form and long-form
      options may be intermixed on the command line.  Options may be given
      in any order, but when incompatible options (e.g., '--output-format'
      and '--exponential') are given in combination, behavior is controlled
      by the last option given.  For example, '-o%.12f -e' gives exponential
      format with the default eight significant digits).

      The following options are available:

      -c, --check
           Check that all units and prefixes defined in the units data file
           reduce to primitive units.  Print a list of all units that cannot
           be reduced.  Also display some other diagnostics about suspicious
           definitions in the units data file.  Only definitions active in
           the current locale are checked.  You should always run 'units'
           with this option after modifying a units data file.

      --check-verbose, --verbose-check
           Like the '--check' option, this option prints a list of units
           that cannot be reduced.  But to help find unit  definitions that
           cause endless loops, it lists the units as they are checked.  If
           'units' hangs, then the last unit to be printed has a bad defini-
           tion.  Only definitions active in the current locale are checked.

      -d ndigits, --digits ndigits
           Set the number of significant digits in the output to the value
           specified (which must be greater than zero).  For example, '-

                                   - 24 -       Formatted:  December 4, 2021

 UNITS(1)                                                           UNITS(1)
                                7 March 2017

           d 12' sets the number of significant digits to 12.  With exponen-
           tial output 'units' displays one digit to the left of the decimal
           point and eleven digits to the right of the decimal point.  On
           most systems, the maximum number of internally meaningful digits
           is 15; if you specify a greater number than your system's max-
           imum, 'units' will print a warning and set the number to the
           largest meaningful value.  To directly set the maximum value,
           give an argument of 'max' (e.g., '-d max').  Be aware, of course,
           that ``significant'' here refers only to the display of numbers;
           if results depend on physical constants not known to this preci-
           sion, the physically meaningful precision may be less than that
           shown.  The '--digits' option conflicts with the '--output-
           format' option.

      -e, --exponential
           Set the numeric output format to exponential (i.e., scientific
           notation), like that used in the Unix 'units' program.  The
           default precision is eight significant digits (seven digits to
           the right of the decimal point); this can be changed with the '-
           -digits' option.  The '--exponential' option conflicts with the
           '--output-format' option.

      -o format, --output-format format
           This option affords complete control over the numeric output for-
           mat using the specified format. The format is a single floating
           point numeric format for the 'printf()' function in the C pro-
           gramming language.  All compilers support the format types 'g'
           and 'G' to specify significant digits, 'e' and 'E' for scientific
           notation, and 'f' for fixed-point decimal.  The ISO C99 standard
           introduced the 'F' type for fixed-point decimal and the 'a' and
           'A' types for hexadecimal floating point; these types are allowed
           with compilers that support them.  The default format is '%.8g';
           for greater precision, you could specify '-o %.15g'.  See Numeric
           Output Format and the documentation for 'printf()' for more
           detailed descriptions of the format specification.  The '--
           output-format' option affords the greatest control of the output
           appearance, but requires at least rudimentary knowledge of the
           'printf()' format syntax.  If you don't want to bother with the
           'printf()' syntax, you can specify greater precision more simply
           with the '--digits' option or select exponential format with '--
           exponential'.  The '--output-format' option is incompatible with
           the '--exponential' and '--digits' options.

      -f filename, --file filename
           Instruct 'units' to load the units file filename.  You can
           specify up to 25 units files on the command line.  When you use
           this option, 'units' will load only the files you list on the
           command line; it will not load the standard file or your personal
           units file unless you explicitly list them.  If filename is the
           empty string ('-f ""'), the default units file (or that specified
           by 'UNITSFILE') will be loaded in addition to any others

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 UNITS(1)                                                           UNITS(1)
                                7 March 2017

           specified with '-f'.

      -L logfile, --log logfile
           Save the results of calculations in the file logfile; this can be
           useful if it is important to have a record of unit conversions or
           other calculations that are to be used extensively or in a criti-
           cal activity such as a program or design project.  If logfile
           exits, the new results are appended to the file.  This option is
           ignored when 'units' is used non-interactively.  See Logging Cal-
           culations for a more detailed description and some examples.

      -H filename, --history filename
           Instruct 'units' to save history to filename, so that a record of
           your commands is available for retrieval across different 'units'
           invocations.  To prevent the history from being saved set
           filename to the empty string ('-H ""').  This option has no
           effect if readline is not available.

      -h, --help
           Print out a summary of the options for 'units'.

      -m, --minus
           Causes '-' to be interpreted as a subtraction operator.  This is
           the default behavior.

      -p, --product
           Causes '-' to be interpreted as a multiplication operator when it
           has two operands.  It will act as a negation operator when it has
           only one operand: '(-3)'.  By default '-' is treated as a sub-
           traction operator.

           Causes '*' to have the old-style precedence, higher than the pre-
           cedence of division so that '1/2*3' will equal '1/6'.

           Forces '*' to have the new (default) precedence that follows the
           usual rules of algebra: the precedence of '*' is the same as the
           precedence of '/', so that '1/2*3' will equal '3/2'.

           Give compact output featuring only the conversion factor.  This
           turns off the '--verbose' option.

      -q, --quiet, --silent
           Suppress prompting of the user for units and the display of
           statistics about the number of units loaded.

      -n, --nolists
           Disable conversion to unit lists.

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      -r, --round
           When converting to a combination of units given by a unit list,
           round the value of the last unit in the list to the nearest

      -S, --show-factor
           When converting to a combination of units specified in a list,
           always show a non-unity factor before a unit that begins with a
           fraction with a unity denominator.  By default, if the unit in a
           list begins with fraction of the form 1|x and its multiplier is
           an integer other than 1, the fraction is given as the product of
           the multiplier and the numerator (e.g., '3|8 in' rather than '3 *
           1|8 in').  In some cases, this is not what is wanted; for exam-
           ple, the results for a cooking recipe might show '3 * 1|2 cup' as
           '3|2 cup'.  With the '--show-factor' option, a result equivalent
           to 1.5 cups will display as '3 * 1|2 cup' rather than '3|2 cup'.
           A user-specified fractional unit with a numerator other than 1 is
           never overridden, however-if a unit list specifies '3|4 cup;1|2
           cup', a result equivalent to 1 1/2 cups will always be shown as
           '2 * 3|4 cup' whether or not the '--show-factor' option is given.

      -s, --strict
           Suppress conversion of units to their reciprocal units.  For
           example, 'units' will normally convert hertz to seconds because
           these units are reciprocals of each other.  The strict option
           requires that units be strictly conformable to perform a conver-
           sion, and will give an error if you attempt to convert hertz to

      -1, --one-line
           Give only one line of output (the forward conversion).  Do not
           print the reverse conversion.  If a reciprocal conversion is per-
           formed then 'units' will still print the ``reciprocal conver-
           sion'' line.

      -t, --terse
           Give terse output when converting units.  This option can be used
           when calling 'units' from another program so that the output is
           easy to parse.  This option has the combined effect of these
           options: '--strict' '--quiet' '--one-line' '--compact'.  When
           combined with '--version' it produces a display showing only the
           program name and version number.

      -v, --verbose
           Give slightly more verbose output when converting units.  When
           combined with the '-c' option this gives the same effect as '--
           check-verbose'.  When combined with '--version' produces a more
           detailed output, equivalent to the '--info' option.

      -V, --version
           Print the program version number, tell whether the 'readline'

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           library has been included, tell whether UTF-8 support has been
           included; give the locale, the location of the default units data
           file, and the location of the personal units data file; indicate
           if the personal units data file does not exist.

      When given in combination with the '--terse' option, the program
      prints only the version number and exits.

      When given in combination with the '--verbose' option, the program,
      the '--version' option has the same effect as the '--info' option

      -I, --info
           Print the information given with the '--version' option, show the
           pathname of the units program, show the status of the 'UNITSFILE'
           and 'MYUNITSFILE' environment variables, and additional informa-
           tion about how 'units' locates the related files.  On systems
           running Microsoft Windows, the status of the 'UNITSLOCALE'
           environment variable and information about the related locale map
           are also given.  This option is usually of interest only to
           developers and administrators, but it can sometimes be useful for

      Combining the '--version' and '--verbose' options has the same effect
      as giving '--info'.

      -U, --unitsfile
           Print the location of the default units data file and exit; if
           the file cannot be found, print ``Units data file not found''.

      -l locale, --locale locale
           Print the information given with the '--version' option, show the
           Force a specified locale such as 'en_GB' to get British defini-
           tions by default.  This overrides the locale determined from sys-
           tem settings or environment variables.  See Locale for a descrip-
           tion of locale format.

    Units Data Files
      The units and prefixes that 'units' can convert are defined in the
      units data file, typically '/usr/share/units/definitions.units'.  If
      you can't find this file, run 'units --version' to get information on
      the file locations for your installation.  Although you can extend or
      modify this data file if you have appropriate user privileges, it's
      usually better to put extensions in separate files so that the defini-
      tions will be preserved if you update 'units'.

      You can include additional data files in the units database using the
      '!include' command in the standard units data file. For example

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         !include    /usr/local/share/units/local.units

      might be appropriate for a site-wide supplemental data file.  The
      location of the '!include' statement in the standard units data file
      is important; later definitions replace earlier ones, so any defini-
      tions in an included file will override definitions before the
      '!include' statement in the standard units data file.  With normal
      invocation, no warning is given about redefinitions; to ensure that
      you don't have an unintended redefinition, run 'units -c' after making
      changes to any units data file.

      If you want to add your own units in addition to or in place of stan-
      dard or site-wide supplemental units data files, you can include them
      in the '.units' file in your home directory.  If this file exists it
      is read after the standard units data file, so that any definitions in
      this file will replace definitions of the same units in the standard
      data file or in files included from the standard data file.  This file
      will not be read if any units files are specified on the command line.
      (Under Windows the personal units file is named 'unitdef.units'.) Run-
      ning 'units -V' will display the location and name of your personal
      units file.

      The 'units' program first tries to determine your home directory from
      the 'HOME' environment variable.  On systems running Microsoft Win-
      dows, if 'HOME' does not exist, 'units' attempts to find your home
      directory from 'HOMEDRIVE', 'HOMEPATH' and 'USERPROFILE'.  You can
      specify an arbitrary file as your personal units data file with the
      'MYUNITSFILE' environment variable; if this variable exists, its value
      is used without searching your home directory.  The default units data
      files are described in more detail in Data Files.

    Defining New Units and Prefixes
      A unit is specified on a single line by giving its name and an
      equivalence.  Comments start with a '#' character, which can appear
      anywhere in a line.  The backslash character ('\') acts as a continua-
      tion character if it appears as the last character on a line, making
      it possible to spread definitions out over several lines if desired.
      A file can be included by giving the command '!include' followed by
      the file's name.  The '!' must be the first character on the line.
      The file will be sought in the same directory as the parent file
      unless you give a full path.  The name of the file to be included can-
      not contain the comment character '#'.

      Unit names must not contain any of the operator characters '+', '-',
      '*', '/', '|', '^', ';', '~', the comment character '#', or
      parentheses.  They cannot begin or end with an underscore ('_'), a
      comma (',') or a decimal point ('.').  The figure dash (U+2012), typo-
      graphical minus (`-'; U+2212), and en dash (`-'; U+2013) are converted
      to the operator '-', so none of these characters can appear in unit
      names.  Names cannot begin with a digit, and if a name ends in a digit
      other than zero, the digit must be preceded by a string beginning with

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      an underscore, and afterwards consisting only of digits, decimal
      points, or commas.  For example, 'foo_2', 'foo_2,1', or 'foo_3.14' are
      valid names but 'foo2' or 'foo_a2' are invalid.  You could define
      nitrous oxide as

         N2O     nitrogen 2  + oxygen

      but would need to define nitrogen dioxide as

         NO_2    nitrogen + oxygen 2

      Be careful to define new units in terms of old ones so that a reduc-
      tion leads to the primitive units, which are marked with '!' charac-
      ters.  Dimensionless units are indicated by using the string
      '!dimensionless' for the unit definition.

      When adding new units, be sure to use the '-c' option to check that
      the new units reduce properly.  If you create a loop in the units
      definitions, then 'units' will hang when invoked with the '-c' option.
      You will need to use the '--check-verbose' option, which prints out
      each unit as it is checked.  The program will still hang, but the last
      unit printed will be the unit that caused the infinite loop.

      If you define any units that contain '+' characters, carefully check
      them because the '-c' option will not catch non-conformable sums.  Be
      careful with the '-' operator as well.  When used as a binary opera-
      tor, the '-' character can perform addition or multiplication depend-
      ing on the options used to invoke 'units'.  To ensure consistent
      behavior use '-' only as a unary negation operator when writing units
      definitions.  To multiply two units leave a space or use the '*'
      operator with care, recalling that it has two possible precedence
      values and may require parentheses to ensure consistent behavior.  To
      compute the difference of 'foo' and 'bar' write 'foo+(-bar)' or even

      Here is an example of a short data file that defines some basic units:

         m       !               # The meter is a primitive unit
         sec     !               # The second is a primitive unit
         rad     !dimensionless  # A dimensionless primitive unit
         micro-  1e-6            # Define a prefix
         minute  60 sec          # A minute is 60 seconds
         hour    60 min          # An hour is 60 minutes
         inch    0.0254 m        # Inch defined in terms of meters
         ft      12 inches       # The foot defined in terms of inches
         mile    5280 ft         # And the mile

      A unit that ends with a '-' character is a prefix.  If a prefix defin-
      ition contains any '/' characters, be sure they are protected by
      parentheses.  If you define 'half- 1/2' then 'halfmeter' would be
      equivalent to '1 / (2 meter)'.

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    Defining Nonlinear Units
      Some unit conversions of interest are nonlinear; for example, tempera-
      ture conversions between the Fahrenheit and Celsius scales cannot be
      done by simply multiplying by conversion factors.

      When you give a linear unit definition such as 'inch 2.54 cm' you are
      providing information that 'units' uses to convert values in inches
      into primitive units of meters.  For nonlinear units, you give a func-
      tional definition that provides the same information.

      Nonlinear units are represented using a functional notation.  It is
      best to regard this notation not as a function call but as a way of
      adding units to a number, much the same way that writing a linear unit
      name after a number adds units to that number.  Internally, nonlinear
      units are defined by a pair of functions that convert to and from
      linear units in the database, so that an eventual conversion to primi-
      tive units is possible.

      Here is an example nonlinear unit definition:

         tempF(x) units=[1;K] domain=[-459.67,) range=[0,) \
                     (x+(-32)) degF + stdtemp ; (tempF+(-stdtemp))/degF + 32

      A nonlinear unit definition comprises a unit name, a formal parameter
      name, two functions, and optional specifications for units, the
      domain, and the range (the domain of the inverse function).  The func-
      tions tell 'units' how to convert to and from the new unit.  To pro-
      duce valid results, the arguments of these functions need to have the
      correct dimensions and be within the domains for which the functions
      are defined.

      The definition begins with the unit name followed immediately (with no
      spaces) by a '(' character.  In the parentheses is the name of the
      formal parameter.  Next is an optional specification of the units
      required by the functions in the definition.  In the example above,
      the 'units=[1;K]' specification indicates that the 'tempF' function
      requires an input argument conformable with '1' (i.e., the argument is
      dimensionless), and that the inverse function requires an input argu-
      ment conformable with 'K'.  For normal nonlinear units definition, the
      forward function will always take a dimensionless argument; in gen-
      eral, the inverse function will need units that match the quantity
      measured by your nonlinear unit.  Specifying the units enables 'units'
      to perform error checking on function arguments, and also to assign
      units to domain and range specifications, which are described later.

      Next the function definitions appear.  In the example above, the
      'tempF' function is defined by

         tempF(x) = (x+(-32)) degF + stdtemp

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      This gives a rule for converting 'x' in the units 'tempF' to linear
      units of absolute temperature, which makes it possible to convert from
      tempF to other units.

      To enable conversions to Fahrenheit, you must give a rule for the
      inverse conversions.  The inverse will be 'x(tempF)' and its defini-
      tion appears after a ';' character.  In our example, the inverse is

         x(tempF) = (tempF+(-stdtemp))/degF + 32

      This inverse definition takes an absolute temperature as its argument
      and converts it to the Fahrenheit temperature.  The inverse can be
      omitted by leaving out the ';' character and the inverse definition,
      but then conversions to the unit will not be possible.  If the inverse
      definition is omitted, the '--check' option will display a warning.
      It is up to you to calculate and enter the correct inverse function to
      obtain proper conversions; the '--check' option tests the inverse at
      one point and prints an error if it is not valid there, but this is
      not a guarantee that your inverse is correct.

      With some definitions, the units may vary.  For example, the defini-

         square(x)       x^2

      can have any arbitrary units, and can also take dimensionless argu-
      ments.  In such a case, you should not specify units.  If a definition
      takes a root of its arguments, the definition is valid only for units
      that yield such a root.  For example,

         squirt(x)       sqrt(x)

      is valid for a dimensionless argument, and for arguments with even
      powers of units.

      Some definitions may not be valid for all real numbers.  In such
      cases, 'units' can handle errors better if you specify an appropriate
      domain and range.  You specify the domain and range as shown below:

         baume(d) units=[1;g/cm^3] domain=[0,130.5] range=[1,10] \
                  (145/(145-d)) g/cm^3 ; (baume+-g/cm^3) 145 / baume

      In this example the domain is specified after 'domain=' with the end-
      points given in brackets.  In accord with mathematical convention,
      square brackets indicate a closed interval (one that includes its end-
      points), and parentheses indicate an open interval (one that does not
      include its endpoints).  An interval can be open or closed on one or
      both ends; an interval that is unbounded on either end is indicated by
      omitting the limit on that end.  For example, a quantity to which
      decibel (dB) is applied may have any value greater than zero, so the
      range is indicated by '(0,)':

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         decibel(x) units=[1;1] range=(0,) 10^(x/10); 10 log(decibel)

      If the domain or range is given, the second endpoint must be greater
      than the first.

      The domain and range specifications can appear independently and in
      any order along with the units specification.  The values for the
      domain and range endpoints are attached to the units given in the
      units specification, and if necessary, the parameter value is adjusted
      for comparison with the endpoints.  For example, if a definition
      includes 'units=[1;ft]' and 'range=[3,)', the range will be taken as
      3 ft to infinity.  If the function is passed a parameter of '900 mm',
      that value will be adjusted to 2.9527559 ft, which is outside the
      specified range.  If you omit the units specification from the previ-
      ous example, 'units' can not tell whether you intend the lower end-
      point to be 3 ft or 3 microfurlongs, and can not adjust the parameter
      value of 900 mm for comparison.  Without units, numerical values other
      than zero or plus or minus infinity for domain or range endpoints are
      meaningless, and accordingly they are not allowed.  If you give other
      values without units then the definition will be ignored and you will
      get an error message.

      Although the units, domain, and range specifications are optional,
      it's best to give them when they are applicable; doing so allows
      'units' to perform better error checking and give more helpful error
      messages.  Giving the domain and range also enables the '--check'
      option to find a point in the domain to use for its point check of
      your inverse definition.

      You can make synonyms for nonlinear units by providing both the for-
      ward and inverse functions; inverse functions can be obtained using
      the '~' operator.  So to create a synonym for 'tempF' you could write

         fahrenheit(x) units=[1;K] tempF(x); ~tempF(fahrenheit)

      This is useful for creating a nonlinear unit definition that differs
      slightly from an existing definition without having to repeat the ori-
      ginal functions.  For example,

         dBW(x)     units=[1;W] range=[0,) dB(x) W ;  ~dB(dBW/W)

      If you wish a synonym to refer to an existing nonlinear unit without
      modification, you can do so more simply by adding the synonym with
      appended parentheses as a new unit, with the existing nonlinear
      unit-without parentheses-as the definition.  So to create a synonym
      for 'tempF' you could write

         fahrenheit()  tempF

      The definition must be a nonlinear unit; for example, the synonym

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         fahrenheit()  meter

      will result in an error message when 'units' starts.

      You may occasionally wish to define a function that operates on units.
      This can be done using a nonlinear unit definition.  For example, the
      definition below provides conversion between radius and the area of a
      circle.  This definition requires a length as input and produces an
      area as output, as indicated by the 'units=' specification.  Specify-
      ing the range as the nonnegative numbers can prevent cryptic error

         circlearea(r) units=[m;m^2] range=[0,)   pi r^2 ; sqrt(circlearea/pi)

    Defining Piecewise Linear Units
      Sometimes you may be interested in a piecewise linear unit such as
      many wire gauges.  Piecewise linear units can be defined by specifying
      conversions to linear units on a list of points.  Conversion at other
      points will be done by linear interpolation.  A partial definition of
      zinc gauge is

         zincgauge[in] 1 0.002, 10 0.02, 15 0.04, 19 0.06, 23 0.1

      In this example, 'zincgauge' is the name of the piecewise linear unit.
      The definition of such a unit is indicated by the embedded '[' charac-
      ter.  After the bracket, you should indicate the units to be attached
      to the numbers in the table.  No spaces can appear before the ']'
      character, so a definition like 'foo[kg meters]' is invalid; instead
      write 'foo[kg*meters]'.  The definition of the unit consists of a list
      of pairs optionally separated by commas.  This list defines a function
      for converting from the piecewise linear unit to linear units.  The
      first item in each pair is the function argument; the second item is
      the value of the function at that argument (in the units specified in
      brackets).  In this example, we define 'zincgauge' at five points.
      For example, we set 'zincgauge(1)' equal to '0.002 in'.  Definitions
      like this may be  more readable  if written using  continuation char-
      acters as

         zincgauge[in] \
              1 0.002  \
             10 0.02   \
             15 0.04   \
             19 0.06   \
             23 0.1

      With the preceding definition, the following conversion can be per-

         You have: zincgauge(10)
         You want: in
             * 0.02

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             / 50
         You have: .01 inch
         You want: zincgauge

      If you define a piecewise linear unit that is not strictly monotonic,
      then the inverse will not be well defined.  If the inverse is
      requested for such a unit, 'units' will return the smallest inverse.

      After adding nonlinear units definitions, you should normally run
      'units --check' to check for errors.  If the 'units' keyword is not
      given, the '--check' option checks a nonlinear unit definition using a
      dimensionless argument, and then checks using an arbitrary combination
      of units, as well as the square and cube of that combination; a warn-
      ing is given if any of these tests fail.  For example,

         Warning: function 'squirt(x)' defined as 'sqrt(x)'
                  failed for some test inputs:
                  squirt(7(kg K)^1): Unit not a root
                  squirt(7(kg K)^3): Unit not a root

      Running 'units --check' will print a warning if a non-monotonic piece-
      wise linear unit is encountered.  For example, the relationship
      between ANSI coated abrasive designation and mean particle size is
      non-monotonic in the vicinity of 800 grit:

         ansicoated[micron] \
              . . .
             600 10.55 \
             800 11.5 \
             1000 9.5 \

      Running 'units --check' would give the error message

         Table 'ansicoated' lacks unique inverse around entry 800

      Although the inverse is not well defined in this region, it's not
      really an error.  Viewing such error messages can be tedious, and if
      there are enough of them, they can distract from true errors.  Error
      checking for nonlinear unit definitions can be suppressed by giving
      the 'noerror' keyword; for the examples above, this could be done as

         squirt(x) noerror domain=[0,) range=[0,) sqrt(x); squirt^2
         ansicoated[micron] noerror \
              . . .

      Use the 'noerror' keyword with caution.  The safest approach after
      adding a nonlinear unit definition is to run 'units --check' and con-
      firm that there are no actual errors before adding the 'noerror' key-

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    Defining Unit List Aliases
      Unit list aliases are treated differently from unit definitions,
      because they are a data entry shorthand rather than a true definition
      for a new unit.  A unit list alias definition begins with '!unitlist'
      and includes the alias and the definition;  for example, the aliases
      included in the standard units data file are

         !unitlist   hms     hr;min;sec
         !unitlist   time    year;day;hr;min;sec
         !unitlist   dms     deg;arcmin;arcsec
         !unitlist   ftin    ft;in;1|8 in
         !unitlist   usvol   cup;3|4 cup;2|3 cup;1|2 cup;1|3 cup;1|4 cup;\
                             tbsp;tsp;1|2 tsp;1|4 tsp;1|8 tsp

      Unit list aliases are only for unit lists, so the definition must
      include a ';'.  Unit list aliases can never be combined with units or
      other unit list aliases, so the definition of 'time' shown above could
      not have been shortened to 'year;day;hms'.

      As usual, be sure to run 'units --check' to ensure that the units
      listed in unit list aliases are conformable.

      By default, 'units' shows results to eight significant digits. You can
      change this with the '--exponential', '--digits', and '--output-
      format' options.  The first sets an exponential format (i.e., scien-
      tific notation) like that used in the original Unix 'units' program,
      the second allows you to specify a different number of significant
      digits, and the last allows you to control the output appearance using
      the format for the 'printf()' function in the C programming language.
      If you only want to change the number of significant digits or specify
      exponential format type, use the '--digits' and '--exponential'
      options.  The '--output-format' option affords the greatest control of
      the output appearance, but requires at least rudimentary knowledge of
      the 'printf()' format syntax. See Invoking Units for descriptions of
      these options.

    Format Specification
      The format specification recognized with the '--output-format' option
      is a subset of that for 'printf()'.  The format specification has the
      form '%'[flags][width]['.'precision]type; it must begin with '%', and
      must end with a floating-point type specifier: 'g' or 'G' to specify
      the number of significant digits, 'e' or 'E' for scientific notation,
      and 'f' for fixed-point decimal.  The ISO C99 standard added the 'F'
      type for fixed-point decimal and the 'a' and 'A' types for hexadecimal
      floating point; these types are allowed with compilers that support
      them.  Type length modifiers (e.g., 'L' to indicate a long double) are
      inapplicable and are not allowed.

      The default format for 'units' is '%.8g'; for greater precision, you
      could specify '-o %.15g'.  The 'g' and 'G' format types use

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      exponential format whenever the exponent would be less than -4, so the
      value 0.000013 displays as '1.3e-005'.  These types also use exponen-
      tial notation when the exponent is greater than or equal to the preci-
      sion, so with the default format, the value 5e7 displays as '50000000'
      and the value 5e8 displays as '5e+008'.  If you prefer fixed-point
      display, you might specify '-o %.8f'; however, small numbers will
      display very few significant digits, and values less than 0.5e-8 will
      show nothing but zeros.

      The format specification may include one or more optional flags: '+',
      ' ' (space), '#', '-', or '0' (the digit zero).  The digit-grouping
      flag ''' is allowed with compilers that support it.  Flags are fol-
      lowed by an optional value for the minimum field width, and an
      optional precision specification that begins with a period (e.g.,
      '.6').  The field width includes the digits, decimal point, the
      exponent, thousands separators (with the digit-grouping flag), and the
      sign if any of these are shown.

      The '+' flag causes the output to have a sign ('+' or '-').  The space
      flag ' ' is similar to the '+' flag, except that when the value is
      positive, it is prefixed with a space rather than a plus sign; this
      flag is ignored if the '+' flag is also given.  The '+' or ' ' flag
      could be useful if conversions might include positive and negative
      results, and you wanted to align the decimal points in exponential
      notation.  The '#' flag causes the output value to contain a decimal
      point in all cases; by default, the output contains a decimal point
      only if there are digits (which can be trailing zeros) to the right of
      the point.  With the 'g' or 'G' types, the '#' flag also prevents the
      suppression of trailing zeros.  The digit-grouping flag ''' shows a
      thousands separator in digits to the left of the decimal point.  This
      can be useful when displaying large numbers in fixed-point decimal;
      for example, with the format '%f',

         You have: mile
         You want: microfurlong
                 * 8000000.000000
                 / 0.000000

      the magnitude of the first result may not be immediately obvious
      without counting the digits to the left of the decimal point.  If the
      thousands separator is the comma (','), the output with the format
      '%'f' might be

         You have: mile
         You want: microfurlong
                 * 8,000,000.000000
                 / 0.000000

      making the magnitude readily apparent.  Unfortunately, few compilers
      support the digit-grouping flag.

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                                7 March 2017

      With the '-' flag, the output value is left aligned within the speci-
      fied field width.  If a field width greater than needed to show the
      output value is specified, the '0' (zero) flag causes the output value
      to be left padded with zeros until the specified field width is
      reached; for example, with the format '%011.6f',

         You have: troypound
         You want: grain
                 * 5760.000000
                 / 0000.000174

      The '0' flag has no effect if the '-' (left align) flag is given.

    Field Width
      By default, the output value is left aligned and shown with the
      minimum width necessary for the specified (or default) precision.  If
      a field width greater than this is specified, the value shown is right
      aligned, and padded on the left with enough spaces to provide the
      specified field width.  A width specification is typically used with
      fixed-point decimal to have columns of numbers align at the decimal
      point; this arguably is less useful with 'units' than with long colum-
      nar output, but it may nonetheless assist in quickly assessing the
      relative magnitudes of results.  For example, with the format

         You have: km
         You want: in
                 * 39370.078740
                 /     0.000025
         You have: km
         You want: rod
                 *   198.838782
                 /     0.005029
         You have: km
         You want: furlong
                 *     4.970970
                 /     0.201168

      The meaning of ``precision'' depends on the format type.  With 'g' or
      'G', it specifies the number of significant digits (like the '--
      digits' option); with 'e', 'E', 'f', or 'F', it specifies the maximum
      number of digits to be shown after the decimal point.

      With the 'g' and 'G' format types, trailing zeros are suppressed, so
      the results may sometimes have fewer digits than the specified preci-
      sion (as indicated above, the '#' flag causes trailing zeros to be

      The default precision is 6, so '%g' is equivalent to '%.6g', and would
      show the output to six significant digits.  Similarly, '%e' or '%f'

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      would show the output with six digits after the decimal point.

      The C 'printf()' function allows a precision of arbitrary size,
      whether or not all of the digits are meaningful.  With most compilers,
      the maximum internal precision with 'units' is 15 decimal digits (or
      13 hexadecimal digits).  With the '--digits' option, you are limited
      to the maximum internal precision; with the '--output-format' option,
      you may specify a precision greater than this, but it may not be mean-
      ingful.  In some cases, specifying excess precision can result in
      rounding artifacts.  For example, a pound is exactly 7000 grains, but
      with the format '%.18g', the output might be

         You have: pound
         You want: grain
                 * 6999.9999999999991
                 / 0.00014285714285714287

      With the format '%.25g' you might get the following:

         You have: 1/3
         You want:
                 Definition: 0.333333333333333314829616256247

      In this case the displayed value includes a series of digits that
      represent the underlying binary floating-point approximation to 1/3
      but are not meaningful for the desired computation.  In general, the
      result with excess precision is system dependent.  The precision
      affects only the display of numbers; if a result relies on physical
      constants that are not known to the specified precision, the number of
      physically meaningful digits may be less than the number of digits

      See the documentation for 'printf()' for more detailed descriptions of
      the format specification.

      The '--output-format' option is incompatible with the '--exponential'
      or '--digits' options; if the former is given in combination with
      either of the latter, the format is controlled by the last option

      Some units have different values in different locations.  The locali-
      zation feature accommodates this by allowing a units data file to
      specify definitions that depend on the user's locale.

      A locale is a subset of a user's environment that indicates the user's
      language and country, and some attendant preferences, such as the for-
      matting of dates.  The 'units' program attempts to determine the
      locale from the POSIX setlocale function; if this cannot be done,
      'units' examines the environment variables 'LC_CTYPE' and 'LANG'.  On

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      POSIX systems, a locale is of the form language'_'country, where
      language is the two-character code from ISO 639-1 and country is the
      two-character code from ISO 3166-1; language is lower case and country
      is upper case. For example, the POSIX locale for the United Kingdom is

      On systems running Microsoft Windows, the value returned by setlo-
      cale() is different from that on POSIX systems; 'units' attempts to
      map the Windows value to a POSIX value by means of a table in the file
      'locale_map.txt' in the same directory as the other data files.  The
      file includes entries for many combinations of language and country,
      and can be extended to include other combinations.  The
      'locale_map.txt' file comprises two tab-separated columns; each entry
      is of the form

         Windows-locale   POSIX-locale

      where POSIX-locale is as described above, and Windows-locale typically
      spells out both the language and country.  For example, the entry for
      the United States is

         English_United States   en_US

      You can force 'units' to run in a desired locale by using the '-l'

      In order to create unit definitions for a particular locale you begin
      a block of definitions in a unit datafile with '!locale' followed by a
      locale name.  The '!' must be the first character on the line.  The
      'units' program reads the following definitions only if the current
      locale matches.  You end the block of localized units with
      '!endlocale'.  Here is an example, which defines the British gallon.

         !locale en_GB
         gallon       4.54609 liter

    Additional Localization
      Sometimes the locale isn't sufficient to determine unit preferences.
      There could be regional preferences, or a company could have specific
      preferences.  Though probably uncommon, such differences could arise
      with the choice of English customary units outside of English-speaking
      countries.  To address this, 'units' allows specifying definitions
      that depend on environment variable settings.  The environment vari-
      ables can be controled based on the current locale, or the user can
      set them to force a particular group of definitions.

      A conditional block of definitions in a units data file begins with
      either '!var' or '!varnot' following by an environment variable name
      and then a space separated list of values.  The leading '!' must
      appear in the first column of a units data file, and the conditional

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                                7 March 2017

      block is terminated by '!endvar'.  Definitions in blocks beginning
      with '!var' are executed only if the environment variable is exactly
      equal to one of the listed values.  Definitions in blocks beginning
      with '!varnot' are executed only if the environment variable does not
      equal any of the list values.

      The inch has long been a customary measure of length in many places.
      The word comes from the latin uncia meaning ``one twelfth,'' referring
      to its relationship with the foot.  By the 20th century, the inch was
      officially defined in English-speaking countries relative to the yard,
      but until 1959, the yard differed slightly among those countries.  In
      France the customary inch, which was displaced in 1799 by the meter,
      had a different length based on a french foot.  These customary defin-
      itions could be accommodated as follows:

         !var INCH_UNIT usa
         yard          3600|3937 m
         !var INCH_UNIT canada
         yard          0.9144 meter
         !var INCH_UNIT uk
         yard          0.91439841 meter
         !var INCH_UNIT canada uk usa
         foot          1|3 yard
         inch          1|12 foot
         !var INCH_UNIT france
         foot          144|443.296 m
         inch          1|12 foot
         line          1|12 inch
         !varnot INCH_UNIT usa uk france canada
         !message Unknown value for INCH_UNIT

      When 'units' reads the above definitions it will check the environment
      variable 'INCH_UNIT' and load only the definitions for the appropriate
      section.  If 'INCH_UNIT' is unset or is not set to one of the four
      values listed then 'units' will run the last block.  In this case that
      block uses the '!message' command to display a warning message.
      Alternatively that block could set default values.

      In order to create default values that are overridden by user settings
      the data file can use the '!set' command, which sets an environment
      variable only if it is not already set;  these settings are only for
      the current 'units' invocation and do not persist.  So if the example
      above were preceded by '!set INCH_UNIT france' then this would make
      'france' the default value for 'INCH_UNIT'.  If the user had set the
      variable in the environment before invoking 'units', then 'units'

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                                7 March 2017

      would use the user's value.

      To link these settings to the user's locale you combine the '!set'
      command with the '!locale' command.  If you wanted to combine the
      above example with suitable locales you could do by preceding the
      above definition with the following:

         !locale en_US
         !set INCH_UNIT usa
         !locale en_GB
         !set INCH_UNIT uk
         !locale en_CA
         !set INCH_UNIT canada
         !locale fr_FR
         !set INCH_UNIT france
         !set INCH_UNIT france

      These definitions set the overall default for 'INCH_UNIT' to 'france'
      and set default values for four locales appropriately.  The overall
      default setting comes last so that it only applies when 'INCH_UNIT'
      was not set by one of the other commands or by the user.

      If the variable given after '!var' or '!varnot' is undefined then
      'units' prints an error message and ignores the definitions that fol-
      low.  Use '!set' to create defaults to prevent this situation from
      arising.  The '-c' option only checks the definitions that are active
      for the current environment and locale, so when adding new definitions
      take care to check that all cases give rise to a well defined set of

      The 'units' program uses the following environment variables:

      HOME Specifies the location of your home directory; it is used by
           'units' to find a personal units data file '.units'.  On systems
           running Microsoft Windows, the file is 'unitdef.units', and if
           'HOME' does not exist, 'units' tries to determine your home
           directory from the 'HOMEDRIVE' and 'HOMEPATH' environment vari-
           ables; if these variables do not exist, units finally tries
           'USERPROFILE'-typically 'C:\Users\username' (Windows Vista and
           Windows 7) or 'C:\Documents and Settings\username' (Windows XP).

           Checked to determine the locale if 'units' cannot obtain it from
           the operating system.  Sections of the standard units data file
           are specific to certain locales.

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           Specifies your personal units data file.  If this variable
           exists, 'units' uses its value rather than searching your home
           directory for '.units'.  The personal units file will not be
           loaded if any data files are given using the '-f' option.

           Specifies the pager to use for help and for displaying the con-
           formable units.  The help function browses the units database and
           calls the pager using the '+n'n syntax for specifying a line
           number.  The default pager is 'more'; 'PAGER' can be used to
           specify alternatives such as 'less', 'pg', 'emacs', or 'vi'.

           Set to either 'US' or 'GB' to choose United States or British
           volume definitions, overriding the default from your locale.

           Specifies the units data file to use (instead of the default).
           You can only specify a single units data file using this environ-
           ment variable.  If units data files are given using the '-f'
           option, the file specified by 'UNITSFILE' will be not be loaded
           unless the '-f' option is given with the empty string ('units -
           f ""').

           Windows only; this variable has no effect on Unix-like systems.
           Specifies the units locale map file to use (instead of the
           default).  This variable seldom needs to be set, but you can use
           it to ensure that the locale map file will be found if you
           specify a location for the units data file using either the '-f'
           option or the 'UNITSFILE' environment variable, and that location
           does not also contain the locale map file.

      The 'units' program uses two default data files: 'definitions.units'
      and 'currency.units'.  The program can also use an optional personal
      units data file '.units' ('unitdef.units' under Windows) located in
      the user's home directory.  The personal units data file is described
      in more detail in Units Data Files.

      On Unix-like systems, the data files are typically located in
      '/usr/share/units' if 'units' is provided with the operating system,
      or in '/usr/local/share/units' if 'units' is compiled from the source

      On systems running Microsoft Windows, the files may be in the same
      locations if Unix-like commands are available, a Unix-like file struc-
      ture is present (e.g., 'C:/usr/local'), and 'units' is compiled from
      the source distribution.  If Unix-like commands are not available, a
      more common location is 'C:\Program Files (x86)\GNU\units' (for 64-bit

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                                7 March 2017

      Windows installations) or 'C:\Program Files\GNU\units' (for 32-bit

      If 'units' is obtained from the GNU Win32 Project
      (, the files are commonly in
      'C:\Program Files\GnuWin32\share\units'.

      If the default units data file is not an absolute pathname, 'units'
      will look for the file in the directory that contains the 'units' pro-
      gram; if the file is not found there, 'units' will look in a directory
      '../share/units' relative to the directory with the 'units' program.

      You can determine the location of the files by running 'units --
      version'.  Running 'units --info' will give you additional information
      about the files, how 'units' will attempt to find them, and the status
      of the related environment variables.

      The standard units data file is in Unicode, using UTF-8 encoding.
      Most definitions use only ASCII characters (i.e., code points U+0000
      through U+007F); definitions using non-ASCII characters appear in
      blocks beginning with '!utf8' and ending with '!endutf8'.

      When 'units' starts, it checks the locale to determine the character
      set.  If 'units' is compiled with Unicode support and definitions;
      otherwise these definitions are ignored.  When Unicode support is
      active, 'units' will check every line of all of the units data files
      for invalid or non-printing UTF-8 sequences; if such sequences occur,
      'units' ignores the entire line.  In addition to checking validity,
      'units' determines the display width of non-ASCII characters to ensure
      proper positioning of the pointer in some error messages and to align
      columns for the 'search' and '?' commands.

      At present, 'units' does not support Unicode under Microsoft Windows.
      The UTF-16 and UTF-32 encodings are not supported on any systems.

      If definitions that contain non-ASCII characters are added to a units
      data file, those definitions should be enclosed within '!utf8' ...
      '!endutf8' to ensure that they are only loaded when Unicode support is
      available.  As usual, the '!' must appear as the first character on
      the line.  As discussed in Units Data Files, it's usually best to put
      such definitions in supplemental data files linked by an '!include'
      command or in a personal units data file.

      When Unicode support is not active, 'units' makes no assumptions about
      character encoding, except that characters in the range 00-7F hexade-
      cimal correspond to ASCII encoding.  Non-ASCII characters are simply
      sequences of bytes, and have no special meanings; for definitions in
      supplementary units data files, you can use any encoding consistent
      with this assumption.  For example, if you wish to use non-ASCII char-
      acters in definitions when running 'units' under Windows, you can use

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                                7 March 2017

      a character set such as Windows ``ANSI'' (code page 1252 in the US and
      Western Europe).  You can even use UTF-8, though some messages may be
      improperly aligned, and 'units' will not detect invalid UTF-8
      sequences.  If you use UTF-8 encoding when Unicode support is not
      active, you should place any definitions with non-ASCII characters
      outside '!utf8' ...  '!endutf8' blocks-otherwise, they will be

      Typeset material other than code examples usually uses the Unicode
      minus (U+2212) rather than the ASCII hyphen-minus operator (U+002D)
      used in 'units'; the figure dash (U+2012) and en dash (U+2013) are
      also occasionally used.  To allow such material to be copied and
      pasted for interactive use or in units data files, 'units' converts
      these characters to U+002D before further processing.  Because of
      this, none of these characters can appear in unit names.

      If the 'readline' package has been compiled in, then when 'units' is
      used interactively, numerous command line editing features are avail-
      able.  To check if your version of 'units' includes 'readline', invoke
      the program with the '--version' option.

      For complete information about 'readline', consult the documentation
      for the 'readline' package.  Without any configuration, 'units' will
      allow editing in the style of emacs.  Of particular use with 'units'
      are the completion commands.

      If you type a few characters and then hit ESC followed by '?' then
      'units' will display a list of all the units that start with the char-
      acters typed.  For example, if you type 'metr' and then request com-
      pletion, you will see something like this:

         You have: metr
         metre             metriccup         metrichorsepower  metrictenth
         metretes          metricfifth       metricounce       metricton
         metriccarat       metricgrain       metricquart       metricyarncount
         You have: metr

      If there is a unique way to complete a unitname, you can hit the TAB
      key and 'units' will provide the rest of the unit name.  If 'units'
      beeps, it means that there is no unique completion.  Pressing the TAB
      key a second time will print the list of all completions.

      The readline library also keeps a history of the values you enter.
      You can move through this history using the up and down arrows.  The
      history is saved to the file '.units_history' in your home directory
      so that it will persist across multiple 'units' invocations.  If you
      wish to keep work for a certain project separate you can change the
      history filename using the '--history' option.  You could, for exam-
      ple, make an alias for 'units' to 'units --history .units_history' so
      that 'units' would save separate history in the current directory. The

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                                7 March 2017

      length of each history file is limited to 5000 lines.  Note also that
      if you run several concurrent copies of 'units' each one will save its
      new history to the history file upon exit.

      The units program includes currency exchange rates and prices for some
      precious metals in the database.  Of course, these values change over
      time, sometimes very rapidly, and 'units' cannot provide real time
      values.  To update the exchange rates run the 'units_cur', which
      rewrites the files containing the currency rates, typically
      '/usr/share/units/currency.units'.  This program requires 'python' and
      the 'unidecode' package, and must be run with suitable permissions to
      write the file.  To keep the rates updated automatically, run it using
      a cron job on a Unix-like system, or a similar scheduling program on a
      different system.  Currency exchange rates are taken from Time Genie
      ( and precious metals pricing from Packetizer
      (  These sites update once per day, so there is no
      benefit in running the update script more often than daily.  You can
      run 'units_cur' with a filename specified on the command line and it
      will write the data to that file.  If you give '-' for the file it
      will write to standard output.

      unit definition
           Define a regular unit.

      prefix- definition
           Define a prefix.

 range=[y1,y2] definition(var) ; inverse(funcname)
      funcname(var) noerror units=[in-units,out-units] domain=[x1,x2]
           Define a nonlinear unit or unit function.  The four optional key-
           words 'noerror', 'units=', 'range=' and 'domain=' can appear in
           any order.  The definition of the inverse is optional.

      tabname[out-units] noerror pair-list
           Define a piecewise linear unit.  The pair list gives the points
           on the table listed in ascending order.  The 'noerror' keyword is

           End a block of definitions beginning with '!locale'

           End a block of definitions begun with '!utf8'

           End a block of definitions begun with '!var' or '!varnot'

      !include file
           Include the specified file.

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                                7 March 2017

      !locale value
           Load the following definitions only of the locale is set to

      !message text
           Display text when the database is read unless the quiet option
           ('-q') is enabled.

      !set variable value
           Sets the environment variable, variable, to the specified value
           only if it is not already set.

      !unitlist alias definition
           Define a unit list alias.

           Load the following definitions only if 'units' is running with
           UTF-8 enabled.

      !var envar value-list
           Load the block of definitions that follows only if the environ-
           ment variable envar is set to one of the values listed in the
           space-separated value list.  If envar is not set, 'units' prints
           an error message and ignores the block of definitions.

      !varnot envar value-list
           Load the block of definitions that follows only if the environ-
           ment variable envar is set to value that is not listed in the
           space-separated value list.  If envar is not set, 'units' prints
           an error message and ignores the block of definitions.

      @DATAFILE@ - the standard units data file


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