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Manning Equation Flow Calculator

Work out manning equation flow instantly with clear inputs, formula shown and shareable results.

About Manning Equation Flow Calculator

Manning's equation gives open-channel velocity as (1/n) R^(2/3) S^(1/2). With a hydraulic radius of 1.1 m, a slope of 0.002 and n = 0.035 for a natural channel, velocity is 1.36 m/s and discharge 11.4 m3/s. The Froude number below one confirms subcritical flow, where depth controls from downstream.

Formulas

Manning velocity

v = (1/n) x R^(2/3) x S^(1/2)

Discharge

Q = A x v

Worked example

With the default values below, Manning Equation Flow Calculator returns Channel Discharge of 11.437. Change any input above and every figure here is recalculated in your browser.

Inputs used

Flow cross-sectional areaminimum 0 m2
8.4 m2
Hydraulic radiusminimum 0 m
1.1 m
Channel slopeminimum 0
0.002
Manning roughness nminimum 0
0.035

Results

Channel Discharge
11.437 m3/s
Mean Velocity
1.362 m/s
Same Discharge In Litres Per Second
11,437
Froude Number
0.415

Step by step

  1. 1Values used
  2. 2Manning velocity
  3. 3Discharge
  4. 4Channel Discharge= 11.437 m3/s
  5. 5Mean Velocity= 1.362 m/s
  6. 6Same Discharge In Litres Per Second= 11,437
  7. 7Froude Number= 0.415

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What roughness values are typical?

About 0.012 for concrete, 0.025 for a clean earth channel, 0.035 for natural streams and 0.07 or more for heavily vegetated floodplains.

What does the Froude number tell me?

Below one the flow is subcritical and tranquil; above one it is supercritical and fast, and a hydraulic jump forms where the two meet.

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