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Sewer Pipe Sizing Calculator

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

About Sewer Pipe Sizing Calculator

Gravity sewers are sized with Manning's equation, evaluated at full bore, then limited to a proportional depth of 70-75 percent so air can move above the flow. The smallest standard diameter whose limited capacity exceeds the design peak flow is selected, and full bore velocity is checked against the self-cleansing minimum.

Formulas

Manning capacity

Q full = (1/n) x (D/4)^(2/3) x S^(1/2) x pi D² / 4

Selection

Choose the smallest diameter where Q full x fill ratio is at least the design flow

Worked example

With the default values below, Sewer Pipe Sizing Calculator returns Pipe size required of 225. Change any input above and every figure here is recalculated in your browser.

Inputs used

Design peak flowminimum 0.01 L/s
12 L/s
Pipe gradientminimum 0.05 %
1 %
Manning roughness nminimum 0.008
0.013
Maximum proportional depthaccepts 0.1–1
0.75

Results

Pipe size required
225 mm
Full bore capacity of that size
44.90 L/s
Full bore velocity
1.129 m/s
Design flow as share of capacity
26.7% %

Step by step

  1. 1Values used
  2. 2Manning capacity
  3. 3Selection
  4. 4Pipe size required= 225 mm
  5. 5Full bore capacity of that size= 44.90 L/s
  6. 6Full bore velocity= 1.129 m/s
  7. 7Design flow as share of capacity= 26.7 %

Assumptions and limits

  • The result is derived only from the 4 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 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Sewer design must follow the drainage code and authority standards, and be verified by a qualified engineer.

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

Preliminary estimate only. Sewer design must follow the drainage code and authority standards, and be verified by a qualified engineer.

Frequently Asked Questions

How is the Sewer Pipe Sizing figure worked out?

Q full = (1/n) x (D/4)^(2/3) x S^(1/2) x pi D² / 4. Gravity sewers are sized with Manning's equation, evaluated at full bore, then limited to a proportional depth of 70-75 percent so air can move above the flow.

Why not run pipes full?

A sewer running full has no ventilation path, so pressure fluctuations break trap seals upstream and septic conditions and odour develop. It also leaves no capacity for peaks.

What values do I need to enter?

This calculator takes 4 inputs: Design peak flow, Pipe gradient, Manning roughness n, Maximum proportional depth. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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