Vent sizing calculator: developed length and fixture units
A vent is sized by three things at once: the drain it serves, the fixture units on that drain, and how long the vent runs. Here is IPC Table 916.1 as the PlumbCalc Pro vent calculator uses it.
Two rules before the table
A vent can't be smaller than 1¼", and it can't be smaller than half the drain it serves: 1½" on a 3" drain, 2" on a 4" drain.
Developed length is the whole run of the vent. Every foot of pipe counts, plus the equivalent length of the fittings. It isn't the straight-line distance to the roof.
Maximum developed length in feet (IPC Table 916.1)
| Drain / stack | Up to DFU | 1¼" vent | 1½" vent | 2" vent | 3" vent | 4" vent |
|---|---|---|---|---|---|---|
| 1¼" | 2 | 30 | ||||
| 1½" | 8 | 50 | 150 | |||
| 1½" | 10 | 30 | 100 | |||
| 2" | 12 | 75 | 200 | |||
| 2" | 20 | 50 | 150 | |||
| 3" | 10 | 42 | 150 | 1,040 | ||
| 3" | 21 | 32 | 110 | 810 | ||
| 3" | 53 | 27 | 94 | 680 | ||
| 3" | 102 | 25 | 86 | 620 | ||
| 4" | 43 | 35 | 250 | 980 | ||
| 4" | 140 | 27 | 200 | 750 | ||
| 4" | 320 | 23 | 170 | 640 |
The code table also lists 2½" vents. The calculator skips them because they're rare on a rough-in, so it may hand you the next size up, never a smaller one.
Worked example
A 3" stack carrying 18 DFU, with a vent run of 100 feet developed length.
18 DFU falls in the "up to 21" row. A 1½" vent is good for 32 feet, which is short. A 2" vent is good for 110 feet, which covers 100.
Answer: 2" vent, which also clears the half-the-drain rule (1½" minimum on a 3" drain).
Run your own fixture list
The DWV drain and vent calculators are on the PlumbCalc Pro home page. Free, no signup.
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