units - conversion program


    units [-f filename]


    -? --help       Display help text
    -f --file       Use specified definition file
    -W --warranty   Display information about (lack of) warranty
    -C --copying    Display license information
       --version    Display version information


The units program converts quantities expressed in various scales to their equivalents in other scales. The units program can only handle multiplicative or affine scale changes. It works interactively by prompting the user for input:

    You have: meters
    You want: feet
        * 3.2808399
        / 0.3048

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

    You have: meters/s
    You want: furlongs/fortnight
        * 6012.8848
        / 0.00016630952

    You have: 1|2 inch
    You want: cm
        * 1.27
        / 0.78740157

    You have: 85 degF
    You want: degC

Powers of units can be specified using the '^' character as shown in the example, or by simple concatenation: 'cm3' is equivalent to 'cm^3'. Multiplication of units can be specified by using spaces, a dash or an asterisk. Division of units is indicated by the slash ('/'). Note that multiplication has a higher precedence than division, so 'm/s/s' is the same as 'm/s^2' or 'm/s s'. Division of numbers must be indicated using the vertical bar ('|'). To convert half a meter, you would write '1|2 meter'. If you write '1/2 meter' then the units program would interpret that as equivalent to '0.5/meter'. 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

The conversion information is read from a units data file. The default file includes definitions for most familiar units, abbreviations and metric prefixes. Some constants of nature included are:

    pi         ratio of circumference to diameter
    c          speed of light
    e          charge on an electron
    g          acceleration of gravity
    force      same as g
    mole       Avogadro's number
    water      pressure per unit height of water
    mercury    pressure per unit height of mercury
    au         astronomical unit

The unit 'pound' is a unit of mass. Compound names are run together so 'pound force' is a unit of force. The unit 'ounce' is also a unit of mass. The fluid ounce is 'floz'. British units that differ from their US counterparts are prefixed with 'br', and currency is prefixed with its country name: 'belgiumfranc', 'britainpound'. When searching for a unit, if the specified string does not appear exactly as a unit name, then units will try to remove a trailing 's' or a trailing 'es' and check again for a match.

To find out what units are available read the standard units file. If you want to add your own units you can supply your own file. A unit is specified on a single line by giving its name and an equivalence. Be careful to define new units in terms of old ones so that a reduction leads to the primitive units which are marked with '!' characters. The units program will not detect infinite loops that could be caused by careless unit definitions. Comments in the unit definition file begin with a '/' character at the beginning of a line.

Prefixes are defined in the same was as standard units, but with a trailing dash at the end of the prefix name. If a unit is not found even after removing trailing 's' or 'es', then it will be checked against the list of prefixes. Prefixes will be removed until a legal base unit is identified.

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

    m         !a!
    sec       !b!
    micro-    1e-6
    minute    60 sec
    hour      60 min
    inch      0.0254 m
    ft        12 inches
    mile      5280 ft


Mark-Jason Dominus, <>


  • None noted.


This program is copyright (c) M-J. Dominus (1996, 1999).

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version, or under Perl's `Artistic License'.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.