It is a dimensionless number that shows whether flow can be treated as a continuum.

It can be expressed with the following equation:

where;
Kn : Knudsen number
λ : Mean free path [m]
L : Characteristic length [m]
kB : Boltzmann constant (1.3807 × 10-23 [J/K])
T : Temperature [K]
σ : Diameter of molecule [m]
P : Absolute pressure [Pa]

Fluid can be treated as a continuum only when Knudsen number is smaller than 1. In other cases, fluid must be treated as a dilute fluid with the consideration of motions of atoms and molecules.

Knudsen number is named after a Danish physicist Martin Knudsen.