Airbag inflator mass flow and exit gas stagnation temperatures are necessary as input for automotive crash simulation programs. Valid experimental data for temperatures and mass flows from an inflator are difficult to obtain. Therefore, predictive methods that accurately model the inflator physics are important to enable valid computer simulations of the occupant dynamics with the inflating airbag. This paper presents techniques for pyrotechnic inflators as well as for hybrid inflators, which utilize pre-pressurized inert gas in conjunction with a burning pyrotechnic. Semi-empirical methods are presented which use closed test tank and inflator combustion pressure data in conjunction with ballistic and thermodynamic equations to predict the inflator mass flux and exit gas temperatures. Also, analytic methods are presented that use ballistic, gas dynamic and thermodynamic equations to predict the gas exit properties of an inflator. Such analytic methods are useful as design tools when performance predictions are needed. Results from all methods are compared to experimental data where data are available.