The Unidata units utility, udunits, supports conversion of unit specifications
between formatted and binary forms, arithmetic manipulation of unit
specifications, and conversion of values between compatible scales of
measurement. A unit is the amount by which a physical quantity is measured. For
example:

Physical Quantity Possible Unit
_________________ _____________
time weeks
distance centimeters
power watts

This utility works interactively and has two modes. In one mode, both an input
and output unit specification are given, causing the utility to print the
conversion between them. In the other mode, only an input unit specification is
given. This causes the utility to print the definition -- in standard units --
of the input unit.