Schmidt number is a dimensionless number that represents the ratio of kinematic viscosity coefficient of a fluid to the diffusion coefficient of a substance.
It is expressed with the following equation:

where;
Sc : Schmidt number
ν : Kinematic viscosity coefficient [m2/s]
D : Diffusion coefficient [m2/s]

Schmidt number substitutes the thermal diffusion coefficient in Prandtl number by the diffusion coefficient of a substance. When Sc = 1, distributions of concentration and velocity of the substance become similar.

Schmidt number is named after the German engineer Ernst Heinrich Wilhelm Schmidt.