Free study tools · 248 CMR 10.16(11)
Individual Vent Size Checker
How small can a vent be? Under 248 CMR 10.16(11)(a), an individual vent must be at least 1-1/4 inches and at least half the diameter of the drain it serves. Pick the drain size and the checker works it out, then rounds up to a real pipe size.
1. What does the vent serve?
2. Minimum vent size
The rule, worked out
Two tests, and the vent has to pass both: it must be at least 1-1/4 inches, and at least one half the diameter of the drain served. Whichever is bigger wins. If half the drain lands between pipe sizes, go up to the next standard size, because you can't buy a 1-1/4-and-a-bit pipe.
| Drain served | Half the drain | Minimum vent |
|---|
Water closets underground. A water closet vent that is underground or below a basement floor is held to 2 inches at minimum, even though half of a 3-inch drain would otherwise allow 1-1/2 inches. Tick the box above to apply it.
Don't mix this up with vent stacks. This rule is for individual vents. A vent stack has its own minimum: 3 inches under 248 CMR 10.16(2)(a).
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Study aid — confirm with the current 248 CMR and your inspector. Values are from 248 CMR 10.16 as published by Cornell LII (amended effective 12/8/2023) and were checked in September 2026. Developed length and other rules can require a vent larger than the minimum shown here. PressureTest is independent and not affiliated with the Commonwealth of Massachusetts, the Board of State Examiners of Plumbers & Gas Fitters or PSI.