Mean Free Path Calculator
Calculate mean free path, number density, impingement rate, monolayer formation time, and Knudsen number for any gas at any pressure and temperature.
Mean free path (λ) is the average distance a gas molecule travels between collisions. In vacuum systems it determines whether flow is viscous (λ ≪ tube diameter), transitional, or molecular (λ ≫ tube diameter — the Knudsen flow regime). At pressures below ~1 Pa, λ exceeds typical beam path lengths and gas-phase absorption effectively vanishes, making the system suitable for UV and IR optical paths sensitive to atmospheric absorption. This calculator uses kinetic theory (λ = kT/(√2·π·d²·P)) to compute mean free path, molecular number density, surface impingement rate, and Knudsen number for any gas, pressure, and temperature combination.