FPETOOL is a computerized package of relatively simple engineering quations and models. A package of engineering tools useful in estimating potential fire hazard and the response of the space and fire protection systems to the developing hazard is presented. The computations use established engineering relationships. This paper outlines those relationships for the benefit of engineers and others interested in using the tools in the package. A separate report, FPETOOL User's Guide (NISTIR 4439), also is available.