Manning’s equation is the standard tool for designing canals, ditches, culverts and storm drains. Rougher channels (higher n) and flatter slopes carry less water at the same depth.
How to use the Open Channel Flow Calculator
- Channel shape
- Bottom width b
- Flow depth y
- Side slope z (H:1V)
- Channel surface (Manning n)
- Manning n
- Bed slope S (m/m)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the channel shape and surface, enter the dimensions and the bed slope.
Inputs explained
| Field | What to enter |
|---|---|
| Channel shape | Choose one: Rectangular, Trapezoidal, Triangular |
| Bottom width b | Units: m, cm, mm, km, in, ft … |
| Flow depth y | Units: m, cm, mm, km, in, ft … |
| Side slope z (H:1V) | A number |
| Channel surface (Manning n) | Choose one: PVC / plastic, Smooth steel, Concrete (finished), Cast iron … |
| Manning n | A number |
| Bed slope S (m/m) | A number |
What each result means
- Hydraulic radius = [Flow area]÷[Wetted perimeter]
- Discharge Q = [Velocity]×[Flow area]
Example values
How the main result changes when Bottom width b changes and everything else stays at the example values above:
| Bottom width b (m) | Discharge Q (m³/s) |
|---|---|
| 1 | 1.4924 |
| 1.5 | 1.9367 |
| 2 | 2.392 |
| 2.5 | 2.8552 |
| 3 | 3.3241 |
| 4 | 4.2739 |
Formula
V = (1/n)·R_h^{2/3}·S^{1/2} (SI), Q = V·A, R_h = A/P
Worked example
Inputs — Channel shape: Trapezoidal; Bottom width b: 2 m; Flow depth y: 0.8 m; Side slope z (H:1V): 1.5; Channel surface (Manning n): Earth channel, clean; Bed slope S (m/m): 0.001.
Results — Flow area = 2.56 m²; Wetted perimeter = 4.88444 m; Hydraulic radius = 0.52411 m; Velocity = 0.93439 m/s; Discharge Q = 2.39203 m³/s; Froude number = 0.3912.
Key terms
- Uniform flow
- Depth and velocity constant along the channel.
Frequently asked questions
What is Manning's n?
An empirical roughness coefficient for the channel surface.
Is the formula valid for any flow?
For steady, uniform, turbulent flow.
How do I get more capacity?
Increase depth, width or slope, or use a smoother lining.
What is the most efficient trapezoid?
Half of a regular hexagon (z = 1/√3).