Fixture unit

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In plumbing, a Fixture Unit (FU) is a unit measuring the rate of water flow, equal to one cubic foot of water per minute. One cubic foot of water is roughly 7.48 gallons. A Fixture Unit is used in plumbing design for both water supply and waste water.

There are situations where a design provides for more FUs being discharged than being supplied. 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 or large volumes of liquid introduced through human consumption and discharge.

Different fixtures have different flow requirements. 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 be used at the same time. This is called "fixture unit" (FU). The relationship between gallons per minute (gmp) and fixture unit is not constant, but varies with the number of fixture units. For example, 1000 FU is equivalent to 220 gpm, but 2000 FU is not double that, but is only 1.5 times a much, or 330 gpm.