At a specific temperature, elastin-like polypeptides (ELPs) undergo a sharp solubility transition that can be exploited in a variety of applications in biotechnology and medicine. The temperature of the transition varies with ELP sequence, molecular weight, and concentration. We present a single equation of three parameters that quantitatively predicts the transition temperature as a function of ELP length and concentration for an ELP of a fixed composition. This model should be useful both for the design of new ELP sequences that exhibit a desired transition temperature and for the selection of variables to trigger the phase transition of an ELP for a given application.