Manning Flow Calculator
Enter Manning n, hydraulic radius, slope and flow area to compute the open-channel velocity and discharge via Manning's equation.
Input Data
Results
At a glance:Manning's equation empirically relates uniform open-channel flow velocity to channel properties: v = (1/n)·R^(2/3)·S^(1/2), where n is Manning's roughness coefficient (concrete ~0.013, earth ~0.025–0.03, natural streams 0.03–0.05), R the hydraulic radius (A/P) and S the energy slope (≈ bed slope for uniform flow). Discharge Q = A·v. It is the workhorse of open-channel hydraulics for designing channels, culverts and storm sewers. For non-circular sections compute R from area and wetted perimeter. This tool returns v and Q from n, R, S and A.
Formula
Velocity: v = (1/n)·R^(2/3)·S^(1/2)
Discharge: Q = A·v
$$V = \dfrac{1}{n}\,R^{2/3}\,S^{1/2},\quad Q = V\,A$$How to Use
- Enter Manning n, hydraulic radius R, slope S and flow area A.
- The calculator returns velocity v and discharge Q.
Case Studies
Trapezoidal channel
n = 0.025, R = 1.0 m, S = 0.001, A = 5 m².
v = (1/0.025)×1×√0.001 ≈ 1.265 m/s.
Q = 5×1.265 ≈ 6.3 m³/s.
FAQ
How do I choose n?
From tables by surface material and condition: smooth concrete 0.011–0.015, rubble 0.020–0.030, natural channels 0.025–0.060. Use a conservative (larger) value for design.
What is the hydraulic radius?
R = A/P, area divided by wetted perimeter. For a full pipe R = D/4; it captures how much boundary friction the flow sees.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.