In fluid dynamics, the Froude number (named after William Froude) is the reciprocal of the square root of the Richardson number.

It is sometimes called Reech-Froude number after Ferdinand Reech , who introduced it for testing ships and propellers in 1852. Also, a number of other French researchers used this number before Froude.

the Froude number is defined as

where u is a representative speed, g the acceleration due to gravity, and h a representative length scale.

When used in the context of the Boussinesq approximation it is defined as

where g' the reduced gravity (see Boussinesq approximation) and h a representative vertical lengthscale. Strictly, this is known as the densimetric Froude number.

The densimetric Froude number is usually preferred by modellers who wish to nondimensionalize a speed preference to the Richardson number which is more commonly encountered when considering stratified shear layers.

For example, the leading edge of a gravity current moves with a front Froude number of about unity.