Skip to content
ENExplore Calculators

Vent Pipe Sizing Calculator

Work out vent pipe sizing instantly with clear inputs, formula shown and shareable results.

About Vent Pipe Sizing Calculator

A vent must be at least half the diameter of the drain it serves and never smaller than 1-1/4 inch, and it must also carry the fixture unit load it is assigned. Permitted developed length grows steeply with diameter, so a long vent run is often the reason a size is increased rather than the fixture load.

Formulas

Vent size

Vent diameter = max(drain / 2, 1.25 in, size required by fixture unit load)

Length limit

Permitted length scales roughly with diameter^2.5

Worked example

With the default values below, Vent Pipe Sizing Calculator returns Vent size required of 2.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Drain pipe size servedminimum 1 in
4 in
Fixture units on the ventminimum 1 DFU
24 DFU
Developed vent lengthminimum 1 ft
40 ft

Results

Vent size required
2.00 in
Half-of-drain minimum
2.00 in
Maximum developed length for this size
130 ft
Length check
Within the permitted developed length

Step by step

  1. 1Values used
  2. 2Vent size
  3. 3Length limit
  4. 4Vent size required= 2.00 in
  5. 5Half-of-drain minimum= 2.00 in
  6. 6Maximum developed length for this size= 130 ft
  7. 7Length check= Within the permitted developed length

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Vent sizing, length limits and connection rules must be taken from the plumbing code adopted locally.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Vent sizing, length limits and connection rules must be taken from the plumbing code adopted locally.

Frequently Asked Questions

How is the Vent Pipe Sizing figure worked out?

Vent diameter = max(drain / 2, 1.25 in, size required by fixture unit load). A vent must be at least half the diameter of the drain it serves and never smaller than 1-1/4 inch, and it must also carry the fixture unit load it is assigned.

What happens if the vent is undersized?

Pressure swings in the drain break trap seals, letting sewer gas into the building, and slow fixtures because air cannot follow the water down.

What values do I need to enter?

This calculator takes 3 inputs: Drain pipe size served, Fixture units on the vent, Developed vent length. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

Related Calculators

Vent Pipe Sizing Calculator – Calculate Vent size required