The fraction of the rates of termolecular collisions/bimolecular collisions is calculated by an analytical and semianalytical expression. The results are compared with results obtained by classical trajectory calculations on benzene and At, which are used to find directly the number of bimolecular and termolecular collisions as a function of pressure for a given set of initial conditions. The comparison indicates very good agreement for the semianalytical model and very reasonable agreement for the analytical model, both in the range 20-150 atm. Equation 15 in the text is a reliable and convenient "hard sphere" formula for calculating the fraction of termolecular collisions rate as a function of pressure.