Calculation of energy and mass balance within an incinerator is a very important part of designing and/or evaluating the incineration process. This article describes a simple computer model used to calculate an energy and mass balance for a rotary kiln incinerator. The main purpose of the model is to assist U.S. Environmental Protection Agency (EPA) permit writers in evaluating the adequacy of the data submitted by applicants seeking incinerator permits. The calculation is based on the assumption that a thermodynamic equilibrium condition exists within the combustion chamber. Key parameters that the model can calculate include theoretical combustion air, excess air needed for actual combustion cases, flue gas flow rate, and exit temperature.