For sound waves in an enclosure, the mean free path is the average distance the wave travels between reflections off of the enclosure's walls. For a particle, such as a molecule, the mean free path is the average distance the particle travels between collisions with other particles in its ensemble.

The formula for calculating the magnitude of the mean free path depends on the characteristics of the system the particle is in. For particles with a high velocity relative to the velocities of an ensemble of identical particles with random locations, the following relationship applies:

l = (nσ)^-1

Where l is the mean free path, n is the number of particles per unit volume, and σ is the effective cross-sectional area for collision. If, on the other hand, the velocities of the identical particles have a Maxwell distribution of velocities, the following relationship applies:

l = ((√2)nσ)^-1