units  2.22
About: GNU Units converts quantities expressed in various scales to their equivalents in other scales.
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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.

To check that currency conversion update script works, and to verify that units works properly, run 'make check'.

To install the program, run 'make install'.

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 below.

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=' to change the location for the currency file.

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 /bin/../share/units, where /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 doskey.

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 units.man 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 incompatibilities 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 http://ftp.gnu.org/gnu/units/windows/. 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 http://gnuwin32.sourceforge.net/packages/units.htm This port includes readline support.

A Java version of units by Roman Redziejowski roman.redz@swipnet.se is available on SourceForge at http://units-in-java.sourceforge.net/

Two versions are available for Android. Steve Pomeroy has a version based on the above Java version that you can obtain at http:// staticfree.info/projects/units/ and Keith Flowers has compiled the C code for Android: http://apps.keithflower.org/?page_id=6

A Perl version was written by Bob Walton bob@bwalton.com and can be accessed either as a units converting web form or as perl source code from: http://bwalton.com/cgi-bin/myunitscgi.pl

A project called Frink uses a (modified) version of the units database to supply a units-aware programming language. https://futureboy.us/frinkdocs/

Jillian England has created a units definition file that changes energy to mass and seconds to meters: https://github.com/NinerXrayBravoTwoTwo/MassEnergyUnits

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 specifications.

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 'thatdontrequireatrailingspacesoyoucanwrite5'. 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 (adrianm@gnu.org).