In continuum mechanics, the Froude number (Fr) is a dimensionless number defined as the ratio of the flow inertia to the external field (the latter in many applications simply due to gravity). Named after William Froude (;), the Froude number is based on the speed–length ratio as defined by him, and is defined as:

where u₀ is a characteristic flow velocity, g₀ is in general a characteristic external field, and l₀ is a characteristic length. The Froude number has some analogy with the Mach number. In theoretical fluid dynamics the Froude number is not frequently considered since usually the equations are considered in the high Froude limit of negligible external field, leading to homogeneous equations that preserve the mathematical aspects. For example homogeneous Euler equations are conservation equations.