IPC 906.1 · Vent sizing

How to size plumbing vents by developed length

Reading Table 906.1 for stack vents and vent stacks, what developed length really means, and the half-size and long-run rules for individual and branch vents — with a worked example and a free link to the code.

Need the actual table? Read IPC Table 906.1 free at codes.iccsafe.org →

Which rule sizes this vent?

Vent type	What it does	Sized by
Stack vent	The dry extension of a soil or waste stack above the highest branch	Table 906.1 (906.1)
Vent stack	A separate vertical vent running alongside a drainage stack	Table 906.1 (906.1)
Individual, branch, circuit, and relief vents	Vents serving one fixture, a group of fixtures, or relieving a branch	Half-size rule, minimum size, and long-run upsizing (906.2)
Vents for sumps and sewage ejectors	Venting a sealed sump	Their own sizing rules in Section 906 — check your edition
Wet vents, waste stack vents, combination drain and vent	Drains that also carry air	Their own sections in Chapter 9 — not Table 906.1

Our own summary, not the code text. Read IPC Section 906 and Table 906.1 free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ.

How to read Table 906.1, step by step

Step	What you do	Watch out for
1	Find the row for the diameter of the soil or waste stack the vent serves	Use the stack size, not the building drain size
2	Within that stack size, find the row for the total DFU connected to the stack	Use the next row up if your total falls between rows
3	Measure the vent’s developed length — along the pipe, through every offset and fitting, not a straight line	Developed length includes horizontal runs in the attic and the vertical rise
4	Read across the vent-diameter columns and pick the smallest vent whose maximum developed length is at least your measured length	A blank cell means that vent size is not listed for that stack and load — go up a size
5	Confirm the vent is not smaller than the minimum and half-size limits	A long vent on a big stack can push you up a size

Our own summary, not the code text. Read IPC Section 906 and Table 906.1 free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ.

Worked example — individual vent for a water closet on a 3-in. drain (906.2)

Input / step	Value	Source
Drain served	3 in.	Design
Half the drain diameter	1-1/2 in.	Not less than one-half the required drain diameter (906.2)
Code minimum vent size	1-1/4 in. — 1-1/2 in. already exceeds it	906.2
Developed length of this vent	55 ft (runs across the attic to join another vent)	Measured along the pipe
Long-run rule	Vents longer than 40 ft developed length go up one nominal size for the whole run	906.2
Result	2-in. vent for the entire 55 ft	1-1/2 in. upsized one nominal size

Worked example values only. Stack vents and vent stacks are sized with Table 906.1 — read it free at codes.iccsafe.org.

Common mistakes

Mistake	What to do instead
Measuring developed length as a straight line on the plan	Measure along the pipe through every offset, vertical rise, and fitting
Using the building drain size to enter Table 906.1	Enter with the diameter of the stack the vent serves
Forgetting to upsize a long individual or branch vent	Past the long-run threshold, increase one nominal size for the entire length, not just the long part
Reducing the vent through the roof	Keep the full size to the terminal; check local frost-closure rules for vent terminals
Treating a wet vent or combination drain and vent like a dry vent	Those systems have their own sections and load limits
Oversizing the trap arm instead of fixing the vent	Trap-to-vent distance is its own limit (Section 909) — relocate the vent

Our own summary, not the code text. Read IPC Section 906 and Table 906.1 free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ.

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Why vent size depends on length

Read Section 906 and Table 906.1 free on codes.iccsafe.org. ICC offers free read-only access to the 2021 and 2024 International Plumbing Code. This page explains the method; it does not reproduce the table.

Vents let air in and out of the drainage system so water rushing down a stack does not pull the water seal out of the traps. Air has friction too, so the longer and smaller the vent, the less it can move. That is why vent sizing is always tied to developed length — the distance along the pipe, including every offset and rise.

Stack vents and vent stacks come from Table 906.1: enter with the stack diameter and its total DFU, then read across to the smallest vent size whose allowed developed length covers your run. Individual, branch, circuit, and relief vents follow 906.2 instead: at least half the diameter of the drain served, never below the code minimum, and one size larger for the whole run once they get long.

Worked example. A water closet on a 3-inch drain needs at least a 1-1/2-inch individual vent. If that vent snakes 55 ft across the attic before it ties in, it is past the 40-ft developed-length limit, so it goes up one nominal size — 2 inches for the entire run.

Western jurisdictions on the Uniform Plumbing Code (IAPMO) size vents from the UPC’s own drainage-unit table and venting chapter; the length limits differ from the IPC. Start from the load: drainage fixture units and drain sizing. For support spacing, see pipe hanger spacing. Field PM keeps DWV rough-in inspections and photos with the job so the vent routing is on record before the drywall goes up.

FAQ

What is developed length for a plumbing vent?+

Developed length is the length measured along the centerline of the pipe and fittings, including every horizontal run, offset, and vertical rise — not the straight-line distance. Table 906.1 limits the developed length of stack vents and vent stacks for each combination of stack size, load, and vent size.

How do I size a vent stack?+

Enter IPC Table 906.1 with the diameter of the soil or waste stack and the total drainage fixture units on it, then read across to the smallest vent diameter whose maximum developed length covers your measured length. Read the table free at codes.iccsafe.org.

How small can an individual vent be?+

Under IPC 906.2, individual, branch, circuit, and relief vents must be at least half the required diameter of the drain they serve and never smaller than the code minimum. A 3-inch water-closet drain, for example, needs at least a 1-1/2-inch individual vent.

Why does a long vent have to be bigger?+

Air moving through a long, small vent loses pressure to friction just like water does. When an individual or branch vent runs past the long-run limit in 906.2, the code requires the next nominal size for the entire developed length so traps are not siphoned.

How does the UPC size vents?+

The Uniform Plumbing Code (IAPMO, common in western states) sizes vents from its own drainage-unit table with maximum vent lengths, plus a half-size rule in its venting chapter. Its values and length limits differ from the IPC, so size with the code your jurisdiction adopted.

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