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Drain Pipe Sizing Calculator

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

About Drain Pipe Sizing Calculator

Gravity drains are sized to run no more than half full so air can move above the water and protect trap seals. Manning's equation, Q = (1.486/n) A R^(2/3) S^(1/2), gives the half-full capacity of each standard size, and the smallest size that carries the fixture unit design flow is selected.

Formulas

Manning capacity

Q = (1.486 / n) x A x R^(2/3) x S^(1/2), evaluated half full

Design flow

GPM ≈ 2.5 x DFU^0.6

Worked example

With the default values below, Drain Pipe Sizing Calculator returns Drain size required of 3.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total drainage fixture unitsminimum 1 DFU
24 DFU
Pipe slopeminimum 0.2 %
2.08 %
Manning roughness nminimum 0.008
0.013

Results

Drain size required
3.0 in
Design flow
16.8 GPM
Capacity of chosen size at half full
28.6 GPM
Capacity used
58.8% %

Step by step

  1. 1Values used
  2. 2Manning capacity
  3. 3Design flow
  4. 4Drain size required= 3.0 in
  5. 5Design flow= 16.8 GPM
  6. 6Capacity of chosen size at half full= 28.6 GPM
  7. 7Capacity used= 58.8 %

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 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Drainage sizing must follow the fixture unit tables and slope requirements of 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. Drainage sizing must follow the fixture unit tables and slope requirements of the plumbing code adopted locally.

Frequently Asked Questions

How is the Drain Pipe Sizing figure worked out?

Q = (1.486 / n) x A x R^(2/3) x S^(1/2), evaluated half full. Gravity drains are sized to run no more than half full so air can move above the water and protect trap seals.

Why is a steeper slope not always better?

Above about 4 percent the water outruns the solids, leaving them stranded in the pipe. The classic quarter inch per foot balances velocity against carrying capacity.

What values do I need to enter?

This calculator takes 3 inputs: Total drainage fixture units, Pipe slope, Manning roughness n. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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