The Knudsen Number (Boltzmann gas) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length could be, for example, the radius of a body in a fluid.
The Knudsen Number helps determine whether statistical mechanics or the continuum mechanics formulation of fluid dynamics should be used to model a situation. If the Knudsen number is near or greater than one, the mean free path of a molecule is comparable to a length scale of a problem, and the continuum assumption of fluid mechanics is no longer a good approximation.
The following formula is used for a Boltzmann gas: `"Kn"=(k_BT)/(sqrt(2)pid^2pL)`, where:
Wikipedia (https://en.wikipedia.org/wiki/Knudsen_number)