The Blake number in fluid mechanics is a nondimensional number showing the ratio of inertial force to viscous force. It is used in momentum transfer in general and in particular for flow of a fluid through beds of solids. It is a generalisation of the Reynolds number for flow through porous media.

Expressed mathematically the Blake number B is:

${\displaystyle B={\frac {u\rho D_{h}}{\mu (1-\epsilon )}}}$

where

ε	=	void fraction
μ	=	dynamic viscosity
ρ	=	fluid density
Dh	=	hydraulic diameter
u	=	flow velocity