Empirical formulae for the compression ratio and speed factor of a single stage turbomolecular pump operating in the molecular flow regime are developed for blade speeds lower than the most probable molecular velocity. These formulae are based on Becker's compression formula derived from drag theory and on Bernhardt's pumping speed formula but are modified and extended to take account of the full range of parameters in the two dimensional infinite blade row model of Kruger. Numerical constants are determined using the direct simulation Monte Carlo method. Results from the latter method are presented as contour plots of transmission probabilities intended to cover cases of interest for design calculations.