WSFU vs DFU — Supply and Drainage Fixture Units Compared
Fixture units let you size pipe without adding up individual flow rates. But the supply side and the drainage side use separate unit systems, separate tables, and separate logic.
What a fixture unit is
A fixture unit is a weighting number that bundles two things: the flow a fixture demands and the probability it is in use at the peak moment. Summing fixture units and reading a table is a shortcut for a full probability calculation.
Water Supply Fixture Units (WSFU)
WSFU represents the load a fixture places on the pressurised supply system. It depends on the fixture’s flow rate, whether it is flush-valve or flush-tank, and whether the use is public or private.
| Fixture | WSFU (private) | WSFU (public) |
|---|---|---|
| Water closet, flush valve | 6 | 10 |
| Water closet, flush tank | 2.5 | 5 |
| Lavatory | 1 | 2 |
| Shower | 2 | 4 |
| Kitchen sink | 2 | 4 |
Total WSFU converts to peak flow (L/s or GPM) through Hunter’s curve, and peak flow sizes the pipe against velocity and friction-loss limits.
Drainage Fixture Units (DFU)
DFU represents the load a fixture places on the gravity drainage system. It is based on discharge volume and rate, and it uses its own tables.
| Fixture | DFU |
|---|---|
| Water closet | 3–4 |
| Lavatory | 1 |
| Shower | 2 |
| Kitchen sink | 2 |
| Floor drain | 2 |
Why the same fixture differs
A flush-valve water closet is 6 WSFU because it briefly demands a very high supply flow at high pressure. On the drain side it is 3–4 DFU because the discharged volume — about 6 litres — is modest and the drain only has to carry it away by gravity. The two numbers answer different questions.
Using them
- Build one fixture schedule; record WSFU and DFU columns side by side.
- Sum WSFU per supply branch → peak flow → supply pipe size.
- Sum DFU per drain branch and stack → drain and stack pipe size, with minimum slope.
- Size vents from the drain fixture-unit load and developed length.
EngEst Pro’s Plumbing Calculator keeps both fixture-unit systems in one schedule — add a fixture once and it contributes WSFU to the supply sizing and DFU to the drainage sizing, then outputs supply pipe, drainage pipe, fittings, and a full plumbing BOM.
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