Froude number, Fr, is a dimensionless group which occurs frequently in the study of hydraulic phenomena involving a free surface.

where u is velocity, g acceleration due to gravity and L height. It represents the ratio of momentum force to gravitational force.

For physical modeling, for example, in naval architecture, Froude number is maintained constant by reducing the liquid velocity in the model in proportion to the square root of the linear dimension.

Often the term Froude number is used for the ratio u/(gL)^{1/2}. This velocity ratio is also called Boussinesq Number.