Manning Flow Calculator
Calculate open-channel flow velocity and discharge with Manning's equation, for rectangular, trapezoidal or triangular channels — enter depth, bottom width, side slope, roughness and bed slope.
Input
SI metric enters lengths in metres and reports discharge in m³/s; US customary uses feet and ft³/s.
Prismatic cross-section shape of the channel.
Channel bottom width (metres in SI, feet in US customary). Ignored for a triangular channel.
Depth of flow (metres in SI, feet in US customary).
Typical values: concrete 0.013, earth (clean) 0.022, gravel 0.025, natural stream 0.035.
Longitudinal bed slope, as a percentage (e.g. 0.5 for a 0.5% slope, meaning 0.5 units of drop per 100 units of run).
Output
| Metric | Value |
|---|---|
| No data yet | |
One-line overview of velocity and discharge.
Guides
The Manning Flow Calculator solves Manning's equation for steady, uniform flow in an open channel — a culvert, ditch, canal or natural stream. Enter the channel geometry, Manning's roughness coefficient and the bed slope, and it computes the flow area, wetted perimeter, hydraulic radius, velocity and discharge (flow rate). It supports rectangular, trapezoidal and triangular (V-shaped) channels, in SI metric or US customary units.
How to use it
- Pick your unit system — SI metric (metres, m³/s) or US customary (feet, ft³/s).
- Choose the channel shape. Rectangular needs a bottom width; triangular needs a side slope; trapezoidal needs both.
- Enter the flow depth, Manning's roughness coefficient (n) and the channel bed slope as a percentage.
Results update instantly. The table shows flow area, wetted perimeter, hydraulic radius, velocity and discharge. A one-line summary gives velocity and discharge for quick copying.
What is Manning's equation?
Manning's equation relates the discharge Q of an open channel to its geometry and roughness:
Q = (k / n) · A · R^(2/3) · S^(1/2)where A is the cross-sectional flow area, R = A / P is the hydraulic radius (P is the wetted perimeter — the length of channel boundary in contact with the water), S is the bed slope (as a decimal, rise/run), n is Manning's roughness coefficient, and k is a unit constant: 1.0 for SI, or 1.486 for US customary units. Velocity follows from continuity: V = Q / A.
What roughness coefficient (n) should I use?
Typical published values: concrete-lined channel 0.013, clean earth channel 0.022, gravel channel 0.025, and a natural stream with weeds and stones 0.035-0.045. Rougher surfaces slow the flow for the same slope and geometry.
How is the geometry computed for each shape?
- Rectangular:
A = b·y,P = b + 2y(b= bottom width,y= flow depth). - Trapezoidal:
A = y·(b + z·y),P = b + 2y·√(1+z²)(z= side slope, horizontal run per unit vertical rise). - Triangular:
A = z·y²,P = 2y·√(1+z²)(no bottom width).
Is my data private?
Completely. Every calculation runs entirely in your browser. Nothing you type is uploaded, stored or sent to a server, so you can run as many channel checks as you like in total privacy.
More ways to use this tool
REST API
curl -X POST https://api.iotools.cloud/v1/tool/manning-flow-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"unitSystem": "si",
"channelShape": "rectangular",
"bottomWidth": "2",
"flowDepth": "1",
"manningN": "0.013",
"channelSlope": "0.5"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `manning-flow-calculator` tool (Manning Flow Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.
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