The normal procedure for modeling the size and shape evolution of non-spherical crystals is to write a multi-dimensional population balance in which the growth rates of each crystal face are needed as input information. These growth rates can be difficult (or at least tedious) to obtain via direct measurement. Moreover, if the growth rates of some faces are not measured then they cannot be included in the calculation, even if they actually do appear during the time evolution of the process. As an alternate approach, we have developed an ab initio predictive method for estimating the relative growth rate of every possible face on a crystal relative to a reference face. Therefore, a single set of growth rate data for a single crystal face provides the absolute growth rate of every face-eliminating the need for additional growth measurements. A brief outline of the methodology will be presented along with several examples where the approach has been used to successfully predict the evolving shapes of API crystals.