Fixture unit calculator
A fixture unit is not a flow rate. It is a weighting the code assigns to a fixture so that a house full of them can be sized without pretending they all run at once. Supply uses WSFU, drainage uses DFU, and the two are different numbers for the same fixture. Count the fixtures here and both totals fall out.
Pre-filled with a two-bath house. Add or remove fixtures with the buttons.
- Water closet, 1.6 gpf tank2.2 WSFU · 3 DFU2
- Lavatory0.7 WSFU · 1 DFU2
- Bathtub or tub/shower1.4 WSFU · 2 DFU1
- Shower, separate stall1.4 WSFU · 2 DFU1
- Kitchen sink1.4 WSFU · 2 DFU1
- Dishwasher1.4 WSFU · 2 DFU1
- Clothes washer standpipe1.4 WSFU · 2 DFU1
- Laundry sink1.4 WSFU · 2 DFU0
- Bar sink1 WSFU · 1 DFU0
- Hose bibb2.5 WSFU · 0 DFU2
- Floor drain0 WSFU · 2 DFU0
17.8
WSFU — water supply demand
18
DFU — drainage load
A building drain of 2" at 1/8" per foot carries 21 DFU, which covers this load.
| Building drain | Capacity at 1/8" / ft |
|---|---|
| 2" | 21 DFU |
| 3" | 42 DFU |
| 4" | 216 DFU |
| 6" | 1172 DFU |
| 8" | 2500 DFU |
Why the two numbers differ
A water closet with a tank draws hard for a few seconds and then stops: high peak, low duration. On the supply side that is 2.2 WSFU. On the drainage side the same fixture dumps a fixed volume into the drain and is rated 3 DFU. A lavatory is the mirror image — small draw, small discharge, 0.7 WSFU and 1 DFU. A hose bibb takes 2.5 WSFU off the supply and contributes nothing to the drain at all.
Mixing the two is the classic plan-check rejection: a drain sized off the supply total, or a trunk sized off DFU. Keep the prefix on the number every time you write it down.
What the totals are for
The WSFU total feeds Hunter's curve, which converts a simultaneous-use estimate into gallons per minute; that gpm figure is what the pipe size calculator reads against the friction budget. The DFU total is read straight against the drain capacity table at the slope you are running — which is why slope and drain size are one decision, not two.
Hot and cold do not add
Code notes
IPC The DFU values here follow IPC Table 709.1 and the drain capacities IPC Table 710.1(2). UPC publishes its own fixture unit schedule with different values for several fixtures — a UPC jurisdiction will not accept an IPC total. Confirm the adopted code with your authority having jurisdiction (AHJ) before you size anything.