Open Channel Flow Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Fluid calculations are especially sensitive to density, viscosity, geometry, pressure conventions, and unit systems. Open Channel Flow Calculator narrows that problem to the relationship used on this page, making the displayed open-channel discharge easier to audit against the inputs and the governing equation.

What this calculator does

The Open Channel Flow Calculator uses Manning’s coefficient (n), Channel slope (s), Water flow depth (y), Bottom width (b) to estimate the page’s Open-channel discharge from the fluid-mechanics relationship below. It is meant for a defined geometry and property set, so the useful part is not just the headline number but also whether your density, viscosity, dimensions, pressure reference, and flow convention match the model.

How to use it

Start with the fields that actually drive this result: Manning’s coefficient (n), Channel slope (s), Water flow depth (y), Bottom width (b). Keep units consistent with the menus beside the fields and avoid mixing values measured under different conditions. After calculating, change one input at a time if you are comparing scenarios; that makes cause-and-effect much easier to see.

How the calculation works

The page forms trapezoidal channel area and wetted perimeter, obtains hydraulic radius, and applies Manning’s equation Q = (1/n)A Rh^(2/3)S^(1/2). If top width is left blank, the geometry reduces toward a rectangular section.

Example

Using the page’s default example (Manning’s coefficient (n) = 0.013; Channel slope (s) = 0.01; Water flow depth (y) = 1 m; Bottom width (b) = 1 m), the calculator reports Open-channel discharge of 3.698076 m³/s. Change one driving input at a time and confirm the result moves in the direction predicted by the equation; that is a quick way to catch a unit or mode mistake.

How to interpret the result

Interpret the Open-channel discharge within the fluid, geometry, pressure reference, and property values you entered. A numerically plausible answer can still be physically wrong if gauge/absolute pressure, diameter/radius, viscosity type, or unit convention is mismatched, so compare the result with the assumptions as well as the formula.

Limitations and notes

The surface selector does not automatically replace Manning n, and the cross-section/design selectors do not generate every channel geometry. Use a Manning coefficient and widths that actually match the material and section you intend to model.