TradesHow this works
A vent has two limits at once. It can never be smaller than half the drain it serves, with a 1.25 inch floor. And a stack vent has to be big enough to carry its fixture-unit load over its developed length, which the IPC vent table sets. This tool checks both and recommends the larger of the two, then shows how much developed length the chosen size has to spare before it must go up a size.
Size the drain first, then bring its fixture-unit load and the vent run here. On long runs the developed length is usually what forces the next size up, so it is the number to watch.
Common questions
How big does a plumbing vent need to be?
At least half the diameter of the drain it serves, and never smaller than 1.25 inches. On top of that floor, a stack vent is sized from the fixture-unit load it carries and how far it runs to open air, using the IPC vent table. This tool applies both and gives you the larger result.
What is developed length and why does it matter?
Developed length is the real distance the vent travels from the stack to the open air at the roof, following every fitting, not the straight-line height. It matters because the same stack and load can need a bigger vent once the run gets long. That is the flip point the table controls.
Is this IPC or UPC?
These are IPC values (Table 916.2, printed as 906.1 in some state adoptions). Many jurisdictions use the UPC or local amendments, which size vents differently. Always confirm the code and edition your inspector enforces before you buy pipe.
Sources: IPC Table 916.2 (Size and Developed Length of Stack Vents and Vent Stacks) + Section 916.1 (half-diameter, 1.25 in minimum, 40 ft rule); numbered 906.1/906.2 in some adoptions (2021). Engine version 1.0.0. Risk tier 2.