A fixture unit is equal to 1 cubic foot (0.028 m3) of water drained in a 1+1⁄4 inches (32 mm) diameter pipe over one minute.
[2] One cubic foot of water is roughly 7.48 US gallons (28.3 L; 6.23 imp gal).
In order to determine the required size of pipe, an arbitrary unit is used for pipe sizing which takes into account the likelihood that all the fixtures will not be used at the same time.
This occurs in situations where liquids may infiltrate or are added to a draining system, such as might happen in a large sports venue.
Examples of how this could occur include rain water infiltration.