The Nakamura equation is used to generate a direct relation (first-order differential equation) between the Avrami and Ozawa crystallization rate constants. As a result, it is possible to compare isothermal and constant-cooling-rate crystallization data in a consistent manner. This is done for isotactic polypropylene (i-PP) and poly(ethylene terephthalate), based upon data available from the literature. Resulting correlations are obtained for these two materials at atmospheric pressure as well as for i-PP at elevated pressure. The results corroborate that the maximum crystallization rate occurs approximately midway between the glass-transition and melting temperatures. In addition, the results imply that there is no need to explicitly incorporate an induction time when the modelling is based upon the Nakamura equation.