How to use this calculator

Choose the section and enter its dimensions, Manning’s n and the bed slope, then either the depth (to get the flow rate) or the flow rate (to get the normal depth).

Formulas

Manning (SI)V = (1/n)R_h^2/3S^1/2, Q = VA

Manning (US)V = (1.49/n)R_h^2/3S^1/2

Froude numberFr = V/√(gA/T)

Worked example

Trapezoidal channel, b = 3 m, side slope 2:1, depth 1.2 m, n = 0.025, slope 0.0016 (the default)

A = (3 + 2 × 1.2)(1.2) = 6.48 m², P = 3 + 2(1.2)√5 = 8.367 m, R_h = 0.7745 m.
V = (1/0.025)(0.7745)^2/3(0.0016)^1/2 = 1.349 m/s.
Q = 8.74 m³/s; Fr = 0.47, so the flow is subcritical.

Frequently asked questions

Where does n come from?

Tables of Manning roughness: about 0.012 for finished concrete, 0.022 for clean earth, 0.03–0.05 for natural streams with weeds or stones.