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Commands Reference, Volume 5

units Command


Converts units in one measure to equivalent units in another measure.


units- ] [ File ]


The units command converts quantities expressed in one measurement to their equivalents in another. The units command is an interactive command. It prompts you for the unit you want to convert from and the unit you want to convert to. This command only does multiplicative scale changes. That is, it can convert from one value to another only when the conversion involves a multiplication. For example, it cannot convert between degrees Fahrenheit and degrees Celsius because the value of 32 must be added or subtracted in the conversion.

You can specify a quantity as a multiplicative combination of units, optionally preceded by a numeric multiplier.

Indicate powers by entering suffixed positive integers, and indicate division with a / (slash).

The units command recognizes lb as a unit of mass, but considers pound to be the British pound sterling. Compound names are run together (such as lightyear). Prefix British units differing from their American counterparts with br (brgallon, for instance).

The /usr/share/lib/unittab file contains a complete list of the units that the units command uses. You can also define new units in this file. The File parameter may be used to override the values of the standard conversion factors listed in the /usr/share/lib/unittab file. The specified file must follow the same format as the unittab file.

Most familiar units, abbreviations, and metric prefixes are recognized by the units command, as well as the following:

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 head per unit height of water
au Astronomical unit


- Lists the conversion factors contained in the /usr/share/lib/unittab file before you are prompted to enter your conversion.


  1. To display conversion factors for inches to centimeters, enter:

    you have: in
    you want: cm 

    The units command returns the following values:

    * 2.540000e+00 
    / 3.937008e-01

    The output tells you to multiply the number of inches by 2.540000e+00 to get centimeters, and to multiply the number of centimeters by 3.937008e-01 to get inches.

    These numbers are in standard exponential notation, so 3.937008e-01 means 3.937008 x 10-1, which is the same as 0.3937008.

    Note: The second number is always the reciprocal of the first; for example, 2.54 equals 1/0.3937008.
  2. To convert a measurement to different units, enter:

    you have: 5 years
    you want: microsec 

    The units command returns the following values:

    * 1.577846e+14 
    / 6.337753e-15

    The output shows that 5 years equals 1.577846 x 1014 microseconds, and that one microsecond equals 6.337753 x 10-15 years.

  3. To give fractions in measurements, enter:

    you have: 1|3 mi
    you want: km

    The units command returns the following values:

    * 5.364480e-01 
    / 1.864114e+00

    The | (vertical bar) indicates division, so 1|3 means one-third. This shows that one-third mile is the same as 0.536448 kilometers.

  4. To include exponents in measurements, enter:

    you have: 1.2-5 gal
    you want: floz

    The units command returns the following values:

    * 1.536000e-03
    / 6.510417e+02

    The expression 1.2-5 gal is the equivalent of 1.2 x 10-5. Do not type an e before the exponent (that is, 1.2e-5 gal is not valid). This example shows that 1.2 x 10-5 (0.000012) gallons equal 1.536 x 10-3 (0.001536) fluid ounces.

  5. To specify complex units, enter:

    you have: gram centimeter/second2
    you want: kg-m/sec2

    The units command returns the following values:

    * 1.000000e-05
    / 1.000000e+05

    The units gram centimeter/second2 mean "grams x centimeters/second2." Similarly, kg-m/sec2 means "kilograms x meters/sec2," which is often read as "kilogram-meters per seconds squared."

  6. If the units you specify after you have: and you want: are incompatible:

    you have: ft
    you want: lb

    The units command returns the following message and values:

    3.048000e-01 m
    4.535924e-01 kg

    The conformability message means the units you specified cannot be converted. Feet measure length, and pounds measure mass, so converting from one to the other does not make sense. Therefore, the units command displays the equivalent of each value in standard units.

    In other words, this example shows that one foot equals 0.3048 meters and that one pound equals 0.4535924 kilograms. The units command shows the equivalents in meters and kilograms because the command considers these units to be the standard measures of length and mass.


/usr/bin/units Contains the units command.
/usr/share/lib/unittab Lists units that the units command creates as well as units defined by the user.

Related Information

The bc command, dc command.

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