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Manning Pipe Flow Calculator

Estimate the full-bore gravity capacity of a circular pipe using Manning's equation. Enter the internal diameter, roughness and slope, and the tool returns the flow the pipe can carry running just full, plus the velocity and hydraulic radius.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the pipe internal diameter in millimetres.
  2. Enter Manning's roughness n (about 0.010–0.013 for PVC or new concrete) and the pipe slope in m/m.
  3. Read the full-bore flow (m³/s and L/s), the flow velocity and the hydraulic radius, with the steps shown below.

How it works

For a pipe flowing exactly full the flow area is A = π·D²/4 and the wetted perimeter is the full circumference π·D, so the hydraulic radius reduces to R = D/4. Substituting into Manning's equation Q = (1/n)·A·R^(2/3)·S^(1/2) gives the gravity capacity, and dividing by area gives the mean velocity.

Worked example

Worked example. A 300 mm pipe with n = 0.013 on a 0.005 slope: A = π·0.3²/4 = 0.070686 m², R = 0.075 m, Q = (1/0.013)·0.070686·0.075^(2/3)·√0.005 = 0.0684 m³/s ≈ 68.4 L/s at 0.967 m/s.

Common mistakes

  • Treating the result as pressurised (surcharged) flow — this is gravity full-bore capacity only.
  • Entering the diameter in metres or the nominal (outside) size instead of the internal bore in millimetres.
  • Confusing pipe slope (fall per metre) with a percentage grade — 0.5% is S = 0.005.

Frequently asked questions

Is this the same as flow in a partly full pipe?

No. This gives the flow when the pipe runs just full. A partially-full pipe has a different area and wetted perimeter, and a circular section actually peaks at about 94% depth, so partial-flow charts are needed for intermediate depths.

Which n value should I use?

Around 0.010–0.011 for smooth PVC, 0.011–0.013 for new concrete, and 0.013–0.015 or higher for old or rough pipes. Use the value your local drainage code specifies for design.

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