Vent Pipe Sizing Calculator
Size vent pipes based on fixture units and developed length per plumbing code tables.
Results
Visualization
How It Works
Vent pipes allow air into the drainage system to prevent siphoning of traps. Vent size depends on the drain pipe size, total fixture units, and the developed length of the vent pipe.
The Formula
Vent size is determined by code tables based on: drain pipe size, DFU load, and developed vent length.
Variables
- DFU — Drainage fixture units served by the vent
- L_dev — Total developed length of vent pipe
- D_drain — Diameter of the drain pipe being vented
Worked Example
A 3" drain with 12 DFU and 30 ft developed length requires a 1-1/2" or 2" vent depending on the code edition.
Practical Tips
- Vent pipe cannot be smaller than 1-1/4" (or 1-1/2" per some codes)
- Vent must extend at least 6" above roof line
- Wet vents can serve as both drain and vent in some configurations
- Keep vents as short and direct as possible