Manning equation calculator

Enter your values. The answer and the full worked solution update as you type.

Result

Flow rate5.792 m³/s

Velocity1.609 m/s

Hydraulic radius0.6667 m

Step-by-step solution

Rectangular section

A = b·y = 3.6 m²

P = b + 2y = 5.4 m

R = A/P = 0.6667 m

Manning

V = (1/n)·R^2/3·S^1/2 = 1.609 m/s

Q = V·A = 5.792 m³/s

Key formulas

Manning equation: V = (1/n)·R^2/3·S^1/2
Flow rate: Q = A·V
Hydraulic radius: R = A/P (P = wetted perimeter, free surface excluded)
Rectangle: A = b·y, P = b + 2y
Trapezoid (side slope z:1): A = (b + z·y)·y, P = b + 2y·√(1 + z²)
Circular pipe, full or half full: R = D/4
Required slope: S = (n·Q/(A·R^2/3))²

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