The Fourmilab Units Calculator is based upon the GNU Units utility. It supports expressions in the “Convert” and “To” fields in the same syntax as described in the GNU Units manual. If the To field is left blank, the definition of the unit in the Convert field will be shown in terms of fundamental units. Entering “?” in the To field shows all standard units with the same dimensions as the expression in the Convert field. A total of 3460 linear units, 109 nonlinear units, and 109 prefixes are defined.