The conversion information is read from a units data file that
is called definitions.units and is usually located in
the /usr/share/units directory.
If you invoke
units with the -V option, it will print
the location of this file.
file includes definitions for all familiar units, abbreviations and
metric prefixes. It also includes many obscure or archaic units.
Many constants of nature are defined, including these:
pi ratio of circumference to diameter c speed of light e charge on an electron force acceleration of gravity mole Avogadro’s number water pressure per unit height of water Hg pressure per unit height of mercury au astronomical unit k Boltzman’s constant mu0 permeability of vacuum epsilon0 permittivity of vacuum G Gravitational constant mach speed of sound
The standard data file includes atomic masses for all of the elements and numerous other constants. Also included are the densities of various ingredients used in baking so that ‘2 cups flour_sifted’ can be converted to ‘grams’. This is not an exhaustive list. Consult the units data file to see the complete list, or to see the definitions that are used.
The ‘pound’ is a unit of mass. To get force, multiply by the force conversion unit ‘force’ or use the shorthand ‘lbf’. (Note that ‘g’ is already taken as the standard abbreviation for the gram.) The unit ‘ounce’ is also a unit of mass. The fluid ounce is ‘fluidounce’ or ‘floz’. When British capacity units differ from their US counterparts, such as the British Imperial gallon, the unit is defined both ways with ‘br’ and ‘us’ prefixes. Your locale settings will determine the value of the unprefixed unit. 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’, ‘es’. Next units will replace a
trailing ‘ies’ with ‘y’. If that fails,
units will check for a prefix. The database includes all
of the standard metric prefixes. Only one prefix is permitted per
unit, so ‘micromicrofarad’ will fail. However, prefixes can
appear alone with no unit following them, so
‘micro*microfarad’ will work, as will ‘micro microfarad’.
To find out which units and prefixes are available, read the standard units data file, which is extensively annotated.
English customary units differ in various ways in different
regions. In Britain a complex system of volume measurements featured
different gallons for different materials such as a wine gallon and
ale gallon that different by twenty percent. This complexity was
swept away in 1824 by a reform that created an entirely new gallon,
the British Imperial gallon defined as the volume occupied by ten
pounds of water. Meanwhile in the USA the gallon is derived from the
1707 Winchester wine gallon, which is 231 cubic inches. These gallons
differ by about twenty percent. By default if
units runs in
the ‘en_GB’ locale you will get the British volume measures. If
it runs in the ‘en_US’ locale you will get the US volume
measures. In other locales the default values are the US
definitions. If you wish to force different definitions then set the
UNITS_ENGLISH to either ‘US’ or
‘GB’ to set the desired definitions independent of the locale.
Before 1959, the value of a yard (and other units of measure defined in terms of it) differed slightly among English-speaking countries. In 1959, Australia, Canada, New Zealand, the United Kingdom, the United States, and South Africa adopted the Canadian value of 1 yard = 0.9144 m (exactly), which was approximately halfway between the values used by the UK and the US; it had the additional advantage of making 1 inch = 2.54 cm (exactly). This new standard was termed the International Yard. Australia, Canada, and the UK then defined all customary lengths in terms of the International Yard (Australia did not define the furlong or rod); because many US land surveys were in terms of the pre-1959 units, the US continued to define customary surveyors’ units (furlong, chain, rod, and link) in terms of the previous value for the foot, which was termed the US survey foot. The US defined a US survey mile as 5280 US survey feet, and defined a statute mile as a US survey mile. The US values for these units differ from the international values by about 2 ppm.
units program uses the international values for these
units; the US values can be obtained by using either the ‘US’ or
the ‘survey’ prefix. In either case, the simple familiar
relationships among the units are maintained, e.g., 1 ‘furlong’ =
660 ‘ft’, and 1 ‘USfurlong’ = 660 ‘USft’, though the
metric equivalents differ slightly between the two cases.
The ‘US’ prefix or the ‘survey’ prefix can also be used to
obtain the US survey mile and the value of the US yard prior to 1959,
e.g., ‘USmile’ or ‘surveymile’ (but not
‘USsurveymile’). To get the US value of the statute mile, use
either ‘USstatutemile’ or ‘USmile’.
Except for distances that extend over hundreds of miles (such as in the US State Plane Coordinate System), the differences in the miles are usually insignificant:
You have: 100 surveymile - 100 mile You want: inch * 12.672025 / 0.078913984
The pre-1959 UK values for these units can be obtained with the prefix ‘UK’.
In the US, the acre is officially defined in terms of the US survey
units uses a definition based on the international
foot. If you want the official US acre use ‘USacre’ and
similarly use ‘USacrefoot’ for the official US version of that
unit. The difference between these units is about 4 parts per million.