The mean free path (MFP) is the average distance which a molecule of a gas- or vapour travels before it collides with another molecule. At high vacuum, these distances become very large, so the particles collide only with the walls of the vacuum chamber.

Pressure

Quantity per cm^{3}

MFP

1013 mbar

3 x 10^{19}

6,8 x 10^{-5} mm

1 x 10^{-3} mbar

3 x 10^{13}

6,8 cm

1 x 10^{-9} mbar

3 x 10^{7}

6,8 km

The mean free path is proportional to the pressure. At atmospheric pressure the particles collide every 10 thausandths of a millimetre, but at a pressure of 10^{9} mbar they collide only after 68 km. The following formula gives the mean free path for air at 200C at pressure P mbar.