Point defects like vacancies can have a profound effect on the structure of perovskite ceramics, but the exact mechanisms by which they do this are unclear. A predictive model for the pseudocubic lattice constant of perovskites, based solely on published ionic radii data, has been developed and adapted as a model for tolerance factor. These models more consistently predict both pseudocubic lattice constant, hence cell volume, and the tolerance factor than existing methods, thus also more accurately modeling the temperature coefficient of resonant frequency (TCF). The relationship between tolerance factor and TCF is revisited.