The separative work unit (SWU) is a measure of the amount of work done by the centrifuge and has units of mass (typically kilogram separative work unit). The work  necessary to separate a mass  of feed of assay  into a mass  of product assay , and tails of mass  and assay  is expressed in terms of the number of separative work units needed, given by the expression<br />
<br />
:where  is the value function, defined as