A model is described to predict the evaporation of a flammable-liquid spill and the subsequent formation of an explosive layer by mixing of the vapors with surrounding air. The model is intended for use in engineering predictions and is, therefore, designed to be easy to use and to produce answers within a very short time. To achieve these goals, simplifying assumptions have been introduced, the most important being the treatment of the flow field as one-dimensional (i.e., only variations in the vertical direction are taken into account). Special adjustments are made through submodels for effects that would not be addressed with sufficient accuracy by neglecting horizontal gradients. This is done to deal with forced ventilation and to treat the case where the vaporizing pool occupies only a fraction of the floor area of the enclosure. The model has been validated by comparison with available data from vaporizing pools and for diffusing heavy vapors; some of these comparisons are presented. Overall, agreement with experiment is found to be satisfactory and generally on the conservative side. This evaluation has confirmed the suitability of the formulation for use as a component in a comprehensive hazard-analysis approach that includes methods to predict the pressure increase from combustion of the explosive layer. Development of an integrated tool is currently under way.