Lewis number is a dimensionless number that represents the ratio of thermal diffusivity and diffusion coefficient.

It can be obtained by the following equation:

where;
Le : Lewis number
α : Thermal diffusivity [m2/s]
D : Diffusion coefficient [m2/s]

Lewis number is an important parameter in a calculation where both heat and diffusion must be taken into account. When Le = 1, distributions of temperature and concentration become similar.

Lewis number can also be expressed in the following equation using Prandtl number and Schmidt number.

Lewis number is named after the American chemist Warren Kendall Lewis.