Evaporation rate can be calculated as a function of the pressure at which the material evaporates (P_e), the temperature at which the source is heated (T) and the molecular mass of the material which is evaporated (M):
α_e is considered to e aproximately 1. The above formula was calculated by replacing the mass of a molecule of the source material (m) and the flux of evaporated material that leaves the source (J_x):
, where n is the concentration of molecules, ν_x the average velocity, k_B the Boltzmann constant.

The next expressions between m and M and R and k_B were used for calculations:
,   (N_A is the Avogardo number)