Output units are checked for consistency with the input, allowing verification of conversion of complex expressions.

The GNU variant is distributed under the GPL although the FreeBSD project maintains a free fork of units from before the license change.

The original units program has been a standard part of Unix since the early Bell Laboratories versions.

UDUNITS is considered the de facto program and library for variable unit conversion for netCDF files.

Version 2.0, released on 2 July 2012, added the ability to convert to sums of units, such as hours and minutes or feet and inches.

The syntax for defining non-linear units was changed, and added optional domain and range specifications.