Individual vs Common vs Wet Vent

Jump to what you need

Individual Vent (and Branch Vent)

An individual vent serves one fixture trap. A branch vent is the dry pipe that collects individual vents and carries them to a stack vent, vent stack, or the roof. Both are dry. Both are sized by IPC 906.2 from the drain they serve and developed length, or by UPC Table 703.2 DFU and length. That is the calculator.

This is the configuration the code prefers when you have the space: a sanitary tee or combo at the fixture, a dry vent up, grade back to the drain, terminate in the open air. No dual-use pipe. No extra footnotes about which fixtures may sit downstream. If you can individual-vent it, the DFU/length pair is the right pair.

Common Vent

IPC 911 (common vent) lets two fixtures connect at the same level to a double-wye or sanitary tee and share one vent. The vent is still dry above that fitting. The catch is geometry and size: the drain between the two traps and the vent connection has to meet the section, and a water closet is not a free-for-all pairing with whatever is nearby.

UPC has its own common-vent language. The point for this site: you cannot type “8 DFU, 25 ft” into an individual-vent calculator and get a common-vent diameter. The shared fitting, the vertical distance between connections, and which two fixtures they are all matter. Use the adopted common-vent section, not 906.2 alone.

Wet Vent

A wet vent is a pipe that carries waste from one or more fixtures and, at the same time, vents another fixture. IPC 912 (and the bathroom-group wet-vent provisions around it) and UPC 908/909 are permission slips with a list: which fixtures, which pipe sizes, dry-vent size serving the wet section, where the water closet may connect, and that the wet section is still sized as a drain for the DFU it carries.

That last sentence is why the calculator would lie. A 2-inch pipe that is wet-venting a bathroom group is sized as a drain for those DFUs and as a vent under a different row. Half-of-drain / 40-foot individual-vent logic does not know that the pipe is half full of water. Using 906.2 on a wet vent is how you “prove” a 1-1/4 in wet section that Chapter 9 never allowed.

Circuit vents, waste stacks used as vents, and combination waste-and-vent are further special cases. None of them are this DFU + developed-length individual-vent pair. If the isometric shows one pipe doing two jobs, put the calculator down.

Why the Calculator Applies Cleanly Only to Individual / Branch

IPC 906.2 names individual, branch, circuit, and relief vents, then immediately points drain size to Table 710.1(2) and applies the 40-foot upsize. Circuit vents have additional 905/910 conditions; this tool does not claim those. Stack vents use Table 906.1. Wet and common vents have their own sections with fixture lists. The honest product is: dry individual or branch vent, DFU in, developed length in, water closet yes/no, IPC 906.2 out, UPC 703.2 beside it.

That is also why drain slope vs vent sizing told you to open Chapter 9 instead of scaling a 1/4 in/ft drop into a diameter. Chapter 9 is several tables. This calculator is one of them.

FAQ