Drainage and Hydrology
Manning's Equation Calculator
Calculate open-channel velocity and discharge from area, hydraulic radius, slope, and Manning roughness.
Parameter sketch
Open-channel flow section
Labels update with the current input values so the formula symbols stay tied to the geometry.
A: 8R: 1.2S: 0.005n: 0.035
Result
Discharge: 27.12 cfs
Velocity
3.39
Discharge
27.12
Coefficient
1.486
Step-by-step calculation
- Use coefficient 1.486 for imperial units.
- Velocity = (1.486/0.035) x 1.2^(2/3) x sqrt(0.005) = 3.39.
- Discharge = area x velocity = 8 x 3.39 = 27.12.
What this means
Check that the hydraulic radius and roughness are appropriate for the actual channel shape and flow regime.
Formula Used
V = (k/n) R^(2/3) S^(1/2); Q = VA
Step-by-Step Example
For A=8 ft2, R=1.2 ft, S=0.005, and n=0.035, flow is about 28.8 cfs.
Engineering Notes
Check units, assumptions, boundary conditions, groundwater, drainage, and whether the simplified method is appropriate before using a result in any project decision.
FAQ
What is hydraulic radius?
It is flow area divided by wetted perimeter.