The units and prefixes that units can convert are defined in the units data file, typically /usr/share/units/definitions.units. Although you can extend or modify this data file if you have appropriate user privileges, it's usually better to put extensions in separate files so that the definitions will be preserved when you update units.
You can include additional data files in the units database using the ‘!include’ command in the standard units data file. For example
might be appropriate for a site-wide supplemental data file. The location of the ‘!include’ statement in the standard units data file is important; later definitions replace earlier ones, so any definitions in an included file will override definitions before the ‘!include’ statement in the standard units data file. With normal invocation, no warning is given about redefinitions; to ensure that you don't have an unintended redefinition, run ‘units -c’ after making changes to any units data file.
If you want to add your own units in addition to or in place of standard or site-wide supplemental units data files, you can include them in the .units file in your home directory. If this file exists it is read after the standard units data file, so that any definitions in this file will replace definitions of the same units in the standard data file or in files included from the standard data file. This file will not be read if any units files are specified on the command line. (Under Windows the personal units file is named unitdef.units.)
The units program first tries to determine your home directory from the HOME environment variable. On systems running Microsoft Windows, if HOME does not exist, units attempts to find your home directory from HOMEDRIVE and HOMEPATH. Running units -V will display the location and name of your personal units file.
You can specify an arbitrary file as your personal units data file with the MYUNITSFILE environment variable; if this variable exists, its value is used without searching your home directory.