When referring to free surface flows, a supercritical flow has a Froude number > 1 and a subcritical flow has a Froude number <1.

(7–18)

where U is the local fluid velocity, g is gravity and l is a length scale. For shallow flow, the length scale is the depth of the fluid. The term is the surface wave speed.

Supercritical flow has a local velocity greater than the surface wave speed, while subcritical flow has a velocity less than the wave speed. This is analogous to supersonic and subsonic flow where the local fluid velocity is compared to the speed of an infinitesimal pressure wave to determine the flow regime.