DWV drain sizing calculator: fixture units to pipe size
Add up the drainage fixture units, then read the smallest pipe that carries them. Here are the tables the PlumbCalc Pro DWV calculator uses, and two worked examples you can check by hand.
Step 1: fixture units (IPC Table 709.1, private use)
| Fixture | DFU | Minimum trap |
|---|---|---|
| Water closet (1.6 gpf) | 3 | 3" (built in) |
| Lavatory | 1 | 1¼" |
| Shower stall | 2 | 2" |
| Bathtub, with or without shower | 2 | 1½" |
| Kitchen sink, including disposal | 2 | 1½" |
| Dishwasher, separate trap | 2 | 1½" |
| Clothes washer standpipe | 2 | 2" |
| Floor drain | 2 | 2" |
| Laundry sink | 2 | 1½" |
| Bidet | 1 | 1¼" |
Step 2: size the pipe (IPC Table 710.1)
| Pipe | Horizontal fixture branch, max DFU | Stack, 3 branch intervals or less | Building drain at ¼" per ft |
|---|---|---|---|
| 1½" | 3 | 4 | – |
| 2" | 6 | 10 | 21 |
| 3" | 20 | 48 | 42 |
| 4" | 160 | 240 | 216 |
| 6" | 620 | 960 | 840 |
The rule that trips people up: by fixture units alone, one bathroom fits on a 2" branch. But nothing that serves a water closet can be smaller than 3". The calculator applies that floor for you.
Worked example: one bathroom
Water closet 3 + lavatory 1 + tub 2 = 6 DFU.
A 2" horizontal branch carries 6 DFU, so the fixture units alone say 2". The water closet raises it to 3". Stack: 3". Building drain at ¼" per foot: 3".
Worked example: a two-bath house
Two bathrooms (6 + 6) + kitchen sink 2 + dishwasher 2 + clothes washer 2 = 18 DFU.
Horizontal branch: 3" carries 20, so 3". Stack: 3" carries 48, so 3". Building drain at ¼" per foot: 3" carries 42, so 3".
Run your own fixture list
The DWV drain and vent calculators are on the PlumbCalc Pro home page. Free, no signup.
Open the DWV calculator How we check the mathMore tools: vent sizing · DWV drain sizing · gas line sizing · pipe offsets · pipe sizing cheat sheet