The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale.

It is defined as:

where

• T, temperature (K)
• k_B, Boltzmann's constant
• P, total pressure (Pa)
• σ, diameter (m)

See also

• Fluid dynamics
• Mach number
• Reynolds number