Mean Free Path Calculator: Gas Molecules, Vacuum Regimes, and Collisions

Your details

Select a preset gas to load its kinetic diameter and molar mass, or choose Custom to enter your own.
Effective collision diameter of the molecule in picometers. Preset gases fill this automatically.
pm
Molar mass of the gas. Used to compute mean molecular speed from the Maxwell-Boltzmann distribution.
g/mol
Gas temperature. Standard conditions are 25 °C (298.15 K).
°C
Gas pressure. Standard atmospheric pressure is 101,325 Pa (1 atm).
Pa
Mean free pathAtmospheric / low vacuum
66.79 nm

Average distance traveled between successive molecular collisions

Mean free path (nm)66.793nm
Collision frequency6.989 GHz
Mean molecular speed466.8 m/s
Mean time between collisions143.1 ps
Number density2.461e+25 m⁻³
T_K298.15
p_Pa101,325
d_m0
M_kgmol0.02897
66.793 nm
Atmospheric<100Medium vacuum100-100000High vacuum100000-100000000Very high vacuum100000000-100000000000Ultra-high vacuum100000000000+

Mean free path is 66.79 nm - atmospheric (gas behaves as a continuum).

  • At 66.79 nm, molecules collide billions of times per metre of travel. The gas behaves as a continuous fluid - Navier-Stokes equations apply and viscosity is pressure-independent.
  • Each molecule undergoes about 6.989 GHz collisions per second and moves at a mean speed of 466.8 m/s.
  • The Knudsen number (mean free path divided by a characteristic length) tells you which fluid model to use: below 0.01 is continuum flow, 0.01-10 is transitional, above 10 is free-molecular flow.

Next stepTo find the Knudsen number, divide this mean free path by the smallest relevant dimension of your system (pipe diameter, gap width, or particle size).

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