When is it useful?
Froude number compares flow speed with a gravity-related wave-speed scale. It is used in open-channel flow and in wave effects around ships, but those applications may use different characteristic lengths.
Formula and inputs
Enter flow speed v in m/s, gravity g in m/s², and an appropriate length L in m. For a simple wide rectangular channel, water depth can serve as L; for more general channels, check the hydraulic-depth definition.
Open this calculatorWorked example
With v = 3 m/s, g = 9.81 m/s², and L = 5 m, Fr = 3 ÷ √(9.81 × 5) ≈ 0.43.
Interpreting the result
If L is the appropriate depth scale for an open channel, Fr below 1 describes subcritical flow; 1 is critical and above 1 is supercritical.
Limits and cautions
Do not apply open-channel subcritical or supercritical labels unchanged to a Froude number based on ship length. Record the chosen definition of L alongside the result.
Formula and concept reference: U.S. Geological Survey — critical flow and Froude number. We created the sample inputs and calculations for this guide.