**Abstract**

Values for the mean free path

l in air are presented for temperatures 15, 20, 23 and 25°C for both dry air and moist air at relative humidities of 50 and 100%, at atmospheric pressure (1.01325 × 10

^{5} Pa) and at pressure 1 × 10

^{5} Pa. Use is made of an expression which minimizes the dependence on physical ‘constants’ for mean free path:

where μ is the viscosity of air,

p is the density of air,

P is the pressure and μ is a numerical factor equal to 0.4987445. Differences of up to 1.6% were obtained between the values of mean free path at 23°C from this work and that of other workers. Comparison between the Cunningham slip correction factor from the present work and that from other work shows differences up to 1.8%.