An index number that represents the load of a fixture on the drainage system so that the load of various fixtures in various applications can be combined. 

The value is based on the volume or volume rate of drainage discharge from the fixture, the time duration of that discharge, and the average time

between successive uses of the fixture. One d.f.u was originally equated to a drainage flow rate of one cubic footper minute or 7.5 gallons per minute 

through the fixture outlet.