What is a Drainage Fixture Unit?

A DFU is a dimensionless value representing the probable discharge load a fixture places on the drainage system, based on its drain rate and frequency of use. IPC Table 709.1 assigns a DFU to each fixture so a system's total load can be summed.

How does DFU differ from WSFU?

WSFU measures supply-side water demand, while DFU measures drainage discharge load. A fixture can have different supply and drainage fixture-unit values because filling and draining behave differently, so the two are tabulated separately in the IPC.

Why is the stack size different from the branch size?

A vertical stack carries the combined load of every branch interval connected to it, so it usually serves a higher DFU total than any single horizontal branch and is sized from a separate stack column in IPC Table 710.1.

Does drain slope affect the size?

Yes. Horizontal drain capacity in IPC Table 710.1 is given by slope. This tool uses the common 1/8 inch per foot column; steeper slopes can carry slightly more load, but verify against the correct column for your installed slope.

Do I add vent fixture units too?

Vents are sized separately based on the DFU they serve and developed length, per the venting tables. This tool focuses on drain and stack sizing from the DFU total; size the vents in a separate step using the same fixture-unit load.

Drainage Fixture Unit (DFU) Calculator

Sizing drain, waste, and vent piping starts by adding up the drainage load every fixture places on the system. This tool assigns the IPC Table 709.1 Drainage Fixture Unit value to each fixture, totals them, and looks up the minimum drain and stack size from IPC Table 710.1.

How it works

The load total drives both the branch and the stack size:

total DFU      = sum of (fixture count × fixture DFU)   (IPC Table 709.1)
horizontal drain = smallest size where DFU ≤ table max at 1/8"/ft slope
vertical stack   = smallest size where DFU ≤ stack table max

Branch and stack use different columns of IPC Table 710.1 because a stack collects the load of all the branch intervals feeding it, while a horizontal branch carries only the fixtures on that run.

Example and tips

A full bathroom group of a water closet (3 DFU), lavatory (1 DFU), and tub/shower (2 DFU) totals 6 DFU, which a 2 inch horizontal branch handles, though a 3 inch run is required wherever a water closet discharges. Always size the stack from the full combined DFU of every branch it serves, not from a single branch, and size vents separately from the same fixture-unit load.