Lewis Number Calculator

Enter the values in the below Lewis Number Calculator and hit calculate button to get your required value using α/ DAB lewis number ratio.

Lewis Number (Le) = α / Dc

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Use the lewis number calculator online to find the ratio that characterizes fluid flows. This lewis number calculator can be used as a thermal or mass diffusivity calculator as well.

What is Lewis's Number?

Lewis number is a comparison of the relative size of the thermal boundary layer and concentration boundary layer. Meaning it will tell how the fluid will behave if we have conjugate mass and thermal transfer.

Lewis number is often described as “Le” or “nle”. It is a ratio between thermal and mass diffusivity. It has no dimension meaning it is unitless.

Such numbers can provide us with a convenient way to characterize problems when there are several coupled transport mechanisms.

Lewis number ratio:

The ratio of the lewis number is as below:

Lewis number = α/ DAB


  • α is the thermal diffusivity.
  • DAB is the mass diffusivity 

This relation tells that the higher lewis number indicates that the thermal transfer is greater than the mass transfer and the low lewis number indicates the mass transfer is greater than the thermal transfer.
This ratio can be changed from the Schmidt number to the Prandtl number also.

How to calculate Lewis's number?

The method to find this ratio is simple. You have to write the thermal diffusion in the numerator and the mass diffusivity in the denominator and solve. View the example ahead.


Find lewis’s number if

Thermal diffusivity (α) = 15 m2/s
Mass diffusivity (DAB) = 12 m2/s


Lewis number = α/ DAB
Lewis number = 15 / 12
                       = 1.25 

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