According to the kinetic theory of gases, the kinetic energy for a gas at a temperature  is
where  is the mass of one molecule,  is the root-mean-square speed of the molecules, and  is the Boltzmann constant. The average molecular speed can be calculated from the Maxwell speed distribution
as  (or, equivalently, ). The rate  at which a gas of molar mass  effuses (typically expressed as the number of molecules passing through the hole per second) is then
Here  is the gas pressure difference across the barrier,  is the area of the hole,  is the Avogadro constant,  is the gas constant and  is the absolute temperature. Assuming the pressure difference between the two sides of the barrier is much smaller than , the average absolute pressure in the system (i.e. ), it is possible to express effusion flow as a volumetric flow rate as follows:
or
where  is the volumetric flow rate of the gas,  is the average pressure on either side of the orifice, and  is the hole diameter.