The number of gas molecules colliding with a surface each second is given by = 0.25 n v_rms , where is the collision rate of gas molecules, n is the number of molecules per unit volume and v_rms is the average velocity of a gas molecule. Expressing this equation in terms of things one can directly measure:

Where is in molecules/cm² –sec, P is the pressure in torr, M is the molecular weight of the gas molecule and T is the temperature in K. As an example, Nitrogen (N₂) has a molecular weight M = 28. In a chamber with nitrogen at room temperature (293 K) and a pressure of 1 x 10^-7 torr, = 3.88 x 10¹³ molecules/cm²-sec.