A drainage fixture unit, or DFU, is the number plumbing code assigns to each fixture to represent how much wastewater it sends into the drain system. Plumbers add up the DFUs on a pipe, then use a code table to pick the drain size. It measures load, not gallons per minute.
A drainage fixture unit, almost always shortened to DFU, is a number that plumbing code gives to each fixture to describe how much load it puts on the drain system. A small bathroom sink might rate 1 or 2 DFU. A toilet rates more. A bathtub sits in between. The DFU is not a measure of exact gallons. It is a design factor that bundles together how much a fixture drains, how long it drains, and how often it gets used, into one number a plumber can add up.
The point of the system is to size drain pipes correctly without guessing. Every fixture on a given drain line has a DFU value from a code table. The plumber adds up all the DFUs that flow into a pipe, then looks at a second code table that says how many total DFUs a pipe of each size can carry. For example, code tables limit how many fixture units a 2-inch drain can handle before the next size up is required. Add a few more fixtures to that line and the total DFU count may push the pipe to 3 inches. This is how a designer knows a branch needs a 2-inch pipe while the main building drain needs 4 inches, long before anyone glues a single joint.
DFUs matter because an undersized drain is a slow, clog-prone drain, and an oversized one wastes money and can actually drain too fast to carry solids along. The fixture-unit method balances that. It lets code writers turn messy, real-world water use into a simple counting exercise. A remodel that adds a second bathroom, for instance, adds a known number of DFUs to the existing drain, and that total tells the plumber whether the old pipe can handle the new load or has to be upsized.
The same idea extends to venting and to the building sewer, each with its own DFU table. That is why a plumber pulling a permit for new fixtures often talks in fixture units rather than gallons: it is the common currency plumbing code uses to keep drains, vents, and sewers all correctly sized for the real load they will carry.