Manning Equation Open Channel Calculator
Estimate gravity flow in an open channel or a pipe flowing full using the Manning equation.
- R = A ÷ P = 0.4 ÷ 1.8 = 0.2222 m
- v = (1/n) × R2/3 × S1/2 = 0.8924 m/s
- Q = v × A = 0.35698 m³/s
Manning Equation (SI)
Q = v × A, R = A ÷ P
n is the Manning roughness coefficient, R the hydraulic radius (area divided by wetted perimeter, m), S the slope (m/m) and A the flow area (m²).
Worked example
A 1 m wide concrete channel with 0.4 m depth and 0.1 % slope: R = 0.4 ÷ 1.8 = 0.222 m, v = (1/0.013) × 0.2222/3 × 0.0316 = 0.89 m/s, Q = 1,285 m³/h.
Frequently Asked Questions
What is Manning n?
An empirical roughness coefficient; smoother surfaces have lower n and carry more flow for the same slope.
Can I use Manning for partly full pipes?
Yes, with the area and wetted perimeter of the partly full section. This tool covers rectangular channels and full pipes.
Is Manning valid for pressurised pipes?
No. For pressure flow use Darcy Weisbach, as in the pipe pressure drop calculator.
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