There are mainly two regimes gas flows, viscous and molecular flow regimes. In the case of viscous flow regime, the mean free paths of particles are much less than the size of the system (D). In this regime, gas density (pressure) is high enough gas-gas collisions dominate and molecules "drag" one another along in the flow. Collisions with walls will play a secondary role only in limiting the gas flow. This regime typically occurs when D (cm) P (Torr) > 0.5 for air at room temperature.

In the case of molecular flow gas density (pressure) is very low and typically occurs when D (cm) P (Torr) > 0.005 for air at room temperature. The mean free paths of particles are much larger than the size of the system (D). Only few molecule-molecule collisions occur and molecule-chamber wall collisions dominate the flow process. The molecules move independently of one another and are held back by walls.

In between these two regimes when the mean free paths are comparable to size of system (D), a complicated intermediate (transition) flow occurs. Sometimes the Knudsen number (K_n) is used to distinguish between regimes. K_n (dimensionless number) is ratio of the mean free path to the characteristic dimension of the chamber (can be diameter of a pipe, or vacuum chamber). When K_n> 1 then it is the molecular flow regime. When K_n< 0.01 it is the viscous flow regime. In between, the flow characteristics are indeterminate.