The Free Software Foundation's "units" unit conversion and calculation utility.

GNU units converts quantities expressed in various scales to their equivalents
in other scales. Units can only handle multiplicative scale changes. For
example, it cannot convert Celsius to Fahrenheit but it can convert
temperature differences between those temperature scales. 

The units are defined externally in an extensive, well annotated data file
that defines over 2000 units. You can also provide your own data file. 

WWW: http://www.gnu.org/directory/units.html

Note that this port will install the utility with a 'g' prefix,
e.g. gunits, but the texinfo documentation will refer to it without
the 'g' prefix.