Manning's Equation Calculator

The standard formula for flow in open channels

Manning's Equation is the go-to empirical formula for estimating flow in ditches, partially full culverts, and natural streams - the roughness coefficient (n) captures how much the channel surface itself resists flow.

Worked example

For a channel with n=0.013 (smooth concrete), 2 m² cross-sectional area, 0.5m hydraulic radius, and 0.001 slope:

Q = (1/0.013) x 2 x 0.5^(2/3) x 0.001^0.5 = 3.0648 m3/s

Channel Surface	Typical Manning's n
Smooth concrete	0.012 - 0.014
Rough concrete/rubble	0.017 - 0.030
Natural channel with vegetation	0.030 - 0.050+