A semi-empirical model for the calculation of the flow rate of blowby gases from an I.C. engine has been developed. The effect of geometry of the engine was considered. This model was subsequently applied for the cases of three, four and five rings. The blowby flow rate was found to be a hyperbolic function of the number of rings. Calculated blowby flow rates for a model hypothetical engine were compared with experimentally measured values of a diesel engine reported in the literature and were found to be in agreement. The method also enables one to know the pressure variation in each dwell space between the rings with respect to crank angle. Also, a quick and fairly accurate method has been suggested for easy assesment of blowby rate.