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GNU '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; but
it can also handle nonlinear conversions such as Fahrenheit to Celsius
(which may appear to be linear but is actually affine).  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: at the 'You have:' prompt, enter the unit
from which you want to convert; at the subsequent 'You want:' prompt,
enter the unit to which you want to convert. For example,

  You have: ft
  You want: m
          * 0.3048
          / 3.2808399

In other words, 1 foot is equal to 0.3048 meter (exactly), and 1 meter
is equal to approximately 3.2808339 feet.

To quit the program, enter either 'quit' or 'exit' at either the 'You
have:' or 'You want:' prompt.  You can also quit by entering Ctrl-C at
any time.

The program's features are described in detail in the user manual. 

Building and Installation
General installation instructions appear in the file 'INSTALL'.  You
should be able to run './configure' followed by 'make'.  If you give
no options to configure, it will compile units to look for the units
data file in a standard location (probably /usr/local/share) and the
currency file in /usr/local/com.

If you want to use the program without installing, you will need to use
the '-f' option; if you plan to use the program frequently without
installing, you can avoid having to do this by setting
'UNITSFILE=definitions.units' in the environment.  You can also avoid
having to do this by building for a relocatable installation, described

If you wish to change the locations of the files you can invoke
'./configure' with the option '--prefix=<your_prefix>' to set a
different installation location.  You may also choose to use
'--sharedstatedir=<directory>' to change the location for the currency

Building for a Relocatable Installation
By default, the location of the units data file is compiled into the
program as an absolute path name, so its location cannot be changed.  If
you invoke configure by typing './configure --enable-relocation', the
data file location will be compiled into the program as
'definitions.units', and units will search for the data file in the
following places:

 *  In the directory that contains the units executable, typical for
    installations on Windows.

 *  In <anypath>/bin/../share/units, where <anypath>/bin is the
    directory that contains the units executable, typical for
    installations on Unix-like systems.  For example, the default
    installation typically places the executable in /usr/local/bin and
    the data file in /usr/local/share/units; if you wished, you could
    change these locations to /usr/me/bin and /usr/me/share/units
    without recompiling the program.

If configure is run with '--enable-relocation', you can run units
without installing without giving the '-f' option.

If units is invoked with the '-f' option or the environment variable
UNITSFILE is set, that path is used, and it makes no difference whether
configure was run with the '--enable-relocation' option.

GNU readline Library
For full functionality you should have the GNU readline library
installed to provide history and editing of data entry.

Note that MacOS comes with editline, which is similar to GNU readline,
but not similar enough.  When compiling under MacOS the configure
script looks for GNU readline in /opt/local.  If you have installed it
somewhere else then invoke configure as follows:
       ./configure -I<path_to_includes> -L<path_to_libs>
Then the configure script should detect readline and compile it in.        

If you are installing on Windows, readline may not be available;
however, command history and intraline editing are available via the
standard Windows console facilities described in the documentation for

Currency Conversion Updates
You can update currency conversions using the units_cur Python script;
see the user manual for details.

Building on Windows
units can be built from the Windows command prompt using Visual Studio;
see UnitsWin.pdf for details.

units can also be built from the MKS Korn shell using Visual Studio;
see UnitsMKS.pdf for details.

The documentation is provided in texinfo, roff, and text format.  The
roff manual page source is generated automatically from the
texinfo documentation; this produces a readable man page when run
through nroff, but most equations are not included.  The manual page
formats well for printing or as PDF with groff.  A printed manual can
also be generated using 'units.dvi'; use this if you encounter problems
with groff.

The distribution includes three icons that may be useful for
installation in a GUI.  Use the icotool command to extract the png files
from the .ico files.  The icon_ms.png file is suitable for use as a
small button.

Incompatibilities with Unix 'units'
This program has the following incompatibilties with unix 'units':
  * The '-' character is a subtraction operator rather than a multiply
      operator by default.
  * Exponentiation in numbers requires an 'e', so you must write 2.5e-2
      instead of 2.5-2.
  * Prefixes are listed in the units file.
  * GNU 'units' tries the -s, -es, and -ies plural forms.
  * The default output format is slightly different.
  * The units database is much larger and more informative, but with some
      differences. (e.g. 'g' is for gravity in unix 'units' and grams in
      GNU 'units'.)  The comment character has been changed to '#'.

GNU 'units' Extensions
GNU 'units' includes the following extensions:
  * Multiplication can be written with a '*' if desired.
  * Exponents can be written with '^' or '**' in units.
  * Exponents can be larger than 9 if written with '^' or '**'.
  * Sums of units can be converted.
  * The units data file is extensively commented. 
  * Units which measure reciprocal dimensions can be converted.
  * Parentheses for grouping are supported.
  * Functions such as sin, cos, and log are supported.
  * Roots of units and rational exponents can be computed.
  * Nonlinear units conversions are supported. 
  * Conversion to lists of units (e.g. feet and inches) is supported

Windows Binary Distribution
A binary distribution for Windows is available at The executable was built with
Microsoft Visual Studio using Makefile.Win and the same source
files included in the source distribution.  The binary version is
usually the same as that of the current source distribution.  There is
currently no support for UTF-8 or readline; however, command history and
intraline editing are available via the standard Windows console
facilities described in the documentation for doskey.

Other Ports
A port of units 1.87 to Windows is available from the 
This port includes readline support.

A Java version of units by Roman Redziejowski <>
is available on SourceForge at

Two versions are available for Android.  Steve Pomeroy has a version
based on the above Java version that you can obtain at http:// and Keith Flowers has compiled the C
code for Android:

A Perl version was written by Bob Walton <> and can be
accessed either as a units converting web form or as perl source code

A project called Frink uses a (modified) version of the units
database to supply a units-aware programming language.

Jillian England has created a units definition file that changes
energy to mass and seconds to meters:


When updating from 1.x to 2.x:

The name of the personal units file has changed from $HOME/units.dat
to $HOME/.units ($HOME/unitdef.units under Windows).

The format for nonlinear unit definitions has changed.  Run 'units -c'
and add the "units=" keyword in front of any bracketed unit

Ideas the future (may or may not happen):

  * Bundle up the units conversion stuff into a library. 
  * Inflation adjusted currency?
  * Allow multiple definitions of the same unit and resolve the
      correct definition by a conformability check.  (This has
      exponential growth behavior in the number of units typed in!)
  * When a nonconformable units error is given list units the user
      might have meant (e.g. britainpound for pound) by a
      conformability check and string pattern match of some sort. 
      "spelling advice"
  * Allow some way of having units like '$' that don't require a trailing
      space so you can write '$5'.  This could be handled by having a 
      command in the units database that specifies units which automatically
      get a space inserted  after their name.   
  * Have a metacommand in the units datafile that specifies how plurals should
      be tried for this file.  This would allow expansion into other 
      languages.  (Of course, the real work of expanding into other languages
      is writing a units file that is appropriate for the language in question
      and includes local units.  It's not just a translation task.)
      Another thing that could be accomplished here would be translation of
      English words like "cubic" and "per" into their symbolic meanings.
      A command in the units file could indicate that "per" should be 
      substituted into a '/' and "cubic" means the cube the next unit.
      As it stands, "per" is hard coded into the parser.
  * Represent uncertainties in values in the database.

    This program owes a lot to Jeff Conrad who made many helpful suggestions,
    found numerous bugs, and helped me to find the definitions of obscure
    units.  Chris Madsen also made some valuable contributions. 

    The documentation has greatly benefited from the suggestions made by
    Robert Chassell who kindly read several drafts.

    The following people have been particularly helpful in fixing portability
    problems: Kaveh Ghazi, Eric Backus, and Marcus Daniels.

Bug reports and suggestions for improvements should be sent to the author:
Adrian Mariano (