Fixture Unit Calculator (DFU / WSFU)
Convert plumbing fixtures into drainage fixture units (DFU) and water supply fixture units (WSFU) so drains, vents, and supply pipes can be sized from the code tables.
- Identify each fixture List every fixture on the branch or system (lavatory, water closet, shower, kitchen sink, and so on).
- Assign DFU and WSFU values The calculator applies the IPC/UPC per-fixture values (for example, a flush-tank water closet is about 3–4 DFU).
- Sum by branch and total Totals roll up per branch and for the whole system, accounting for private versus public use.
- Size pipe from the total Use the DFU/WSFU total to look up the minimum drain, vent, or supply pipe size in the matching code table.
How many fixture units is a toilet?
Under the IPC, a residential flush-tank water closet is 3 DFU for drainage; under the UPC it is typically counted as 3–4 DFU. A flushometer (commercial) water closet is higher, often 6 DFU. On the water-supply side a tank-type toilet is about 2.2 WSFU. Always use the table that matches your adopted code and whether the use is private or public.
What is the difference between DFU and WSFU?
Drainage fixture units (DFU) measure the load a fixture places on drain and vent piping; water supply fixture units (WSFU) measure the demand it places on the supply system. They are not interchangeable — a fixture has separate DFU and WSFU values, and you size drains from DFU totals and supply pipe from WSFU totals.
How do I convert fixture units to GPM?
Use Hunter’s curve (the IPC water-supply demand chart): fixture-unit totals are converted to probable simultaneous demand in GPM, because not every fixture runs at once. Roughly, 10 WSFU is about 8 GPM and 100 WSFU is about 44 GPM, but the curve is nonlinear — read it from the code table rather than scaling linearly.