Froude Number 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. Froude Number Calculator narrows that problem to the relationship used on this page, making the displayed froude number easier to audit against the inputs and the governing equation.

What this calculator does

The Froude Number Calculator uses Area of cross-section (A), Width of the channel (W), Flow velocity (u), Acceleration due to gravity (g) to estimate the page’s Froude number 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: Area of cross-section (A), Width of the channel (W), Flow velocity (u), Acceleration due to gravity (g). 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

Froude number is Fr = v/√(gDh), using hydraulic depth Dh from the entered geometry. For ordinary open-channel interpretation, Fr 1 supercritical.

Example

Using the page’s default example (Area of cross-section (A) = 1 m²; Width of the channel (W) = 1 m; Flow velocity (u) = 1 m/s; Acceleration due to gravity (g) = 9.80665 m/s²), the calculator reports Froude number of 0.31933. 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

Use Fr chiefly as a flow-regime indicator for the entered hydraulic depth: subcritical, critical, or supercritical behavior has different wave and control characteristics.

Limitations and notes

Real flows can add turbulence, fittings, entrance/exit losses, compressibility, cavitation, surface roughness, temperature-dependent properties, and geometry effects beyond a compact equation. Recheck units and pressure conventions, then use measured data or an appropriate standard for critical design work. For the Froude Number case on this page, keep that check tied to the displayed inputs rather than carrying the same assumption over from a different calculator.