The need for a simple and reliable method for predicting the efficiency of a turbine stage without carrying out a detailed aerodynamic design is enhanced. The results of an optimization study carried on a large number of turbine stages are presented. The turbine stage efficiency is found to be a function of three main parameters: the expansion ratio, defined as the specific volume variation across the turbine in an isentropic process; the dimensional parameter V̇_out/Δhis 1/4 , which accounts for actual turbine dimensions, and the specific speed. The presented method is believed to be useful mainly for nonconventional turbine stages, the efficiency of which cannot be anticipated on previous machines experience.