Manual browser: units(1)

UNITS(1) General Commands Manual UNITS(1)


unitsconversion program


units [-Llqv] [-f filename] [[count] from-unit to-unit]


units converts quantities expression in various scales to their equivalents in other scales. units can only handle multiplicative scale changes. It cannot convert Centigrade to Fahrenheit, for example.

The following options and arguments are supported:

-f filename
Specifies the name of the units data file to load.
-l or -L
List all unit definitions to the standard output, instead of performing any conversions. The result may include error messages and comments, beginning with ‘/’.

With the -l option, unit definitions will be listed in a format almost identical to the the units data file that was loaded, except that comments will be removed, spacing may be changed, and lines may be re-ordered.

With the -L option, all unit definitions will be reduced to a form that depends on only a few primitive units (such as m, kg, sec).

Suppresses prompting of the user for units and the display of statistics about the number of units loaded.
Prints the version number.
[count] from-unit to-unit
Allows a single unit conversion to be done directly from the command line. No prompting will occur. units will print out only the result of this single conversion. Specifying count and from-unit as two separate arguments is equivalent to embedding both parts inside a single from-unit argument, with the parts separated by a space.

units 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

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”. If the user enters 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:

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

“pound” is a unit of mass. Compound names are run together so “poundforce” is a unit of force. 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 the units program will try to remove a trailing “s” or a trailing “es” and check again for a match.

All of these definitions can be read in the standard units file, or you can supply your own file. A unit is specified on a single line by giving its name and an equivalence. One should 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. units will not detect infinite loops that could be caused by careless unit definitions.

Prefixes are defined in the same way as standard units, but with a trailing dash at the end of the prefix name.


the standard units library


Adrian Mariano <> or <>


While units can be used as a calculator for many unit-related computations, caution is required: many computations require additional constant factors deriving from the physics (or chemistry or whatever) of the situation. As these factors are dimensionless, units cannot itself either provide them or warn the user when they have been forgotten. For example, one joule is one kilogram meter squared per second squared, by definition; however, the kinetic energy of a one-kilogram object moving at one meter per second is half a joule, not one joule, because of a dimensionless factor that arises from integration.

Also, some pairs of units that have the same dimensionality are nonetheless used to measure different things and attempting to convert between them may require additional fudge factors or be entirely meaningless. For example, torque and energy have the same dimensionality, but attempting to convert torque in newton-meters to energy in joules is nonsensical. There is no practical way for units to warn about these issues either.


The effect of including a ‘/’ in a prefix is surprising.

Exponents entered by the user can be only one digit. You can work around this by multiplying several terms.

The user must use ‘|’ to indicate division of numbers and ‘/’ to indicate division of symbols. This distinction should not be necessary.

The program contains various arbitrary limits on the length of the units converted and on the length of the data file.

The program should use a hash table to store units so that it doesn't take so long to load the units list and check for duplication.

January 6, 2013 NetBSD 7.0