Work out vent pipe sizing instantly with clear inputs, formula shown and shareable results.
A vent must be at least half the diameter of the drain it serves and never smaller than 1-1/4 inch, and it must also carry the fixture unit load it is assigned. Permitted developed length grows steeply with diameter, so a long vent run is often the reason a size is increased rather than the fixture load.
Vent size
Vent diameter = max(drain / 2, 1.25 in, size required by fixture unit load)
Length limit
Permitted length scales roughly with diameter^2.5
Preliminary estimate only. Vent sizing, length limits and connection rules must be taken from the plumbing code adopted locally.
Vent diameter = max(drain / 2, 1.25 in, size required by fixture unit load). A vent must be at least half the diameter of the drain it serves and never smaller than 1-1/4 inch, and it must also carry the fixture unit load it is assigned.
Pressure swings in the drain break trap seals, letting sewer gas into the building, and slow fixtures because air cannot follow the water down.
This calculator takes 3 inputs: Drain pipe size served, Fixture units on the vent, Developed vent length. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.