Manning Equation Calculator
Reference page for estimating open-channel flow using Manning’s equation for pipes, channels, swales, ditches, and culverts flowing under open-channel conditions.
Manning equation
Q = (1.486 / n) × A × R2/3 × S1/2
Where Q is flow in cubic feet per second, n is Manning’s roughness coefficient, A is flow area in square feet, R is hydraulic radius in feet, and S is energy slope or channel slope in ft/ft.
Input checklist
- Channel shape and dimensions.
- Flow depth or assumed normal depth.
- Manning’s n value based on lining and condition.
- Slope in ft/ft.
- Freeboard, tailwater, inlet/outlet control, and transition conditions where applicable.
Common uses
Channels and swales
Estimate capacity, velocity, and sensitivity to slope or lining roughness.
Storm drain pipes
Approximate full-flow or partial-flow conveyance where open-channel assumptions apply.
Drainage reviews
Screen proposed geometry before detailed hydraulic modeling.