Complemented by an estimating tool spreadsheet based on a fixed set of chemicals to assist in risk estimations, Probability of Ignition of a Released Flammable Mass converts a "best guess" to a calculated value based on available information and current technology. The text documents and explains the science and background of the technology–based approach. The tool, when populated with appropriate data, yields an estimate of the probability that a defined release of a flammable material will ignite if exposed to an ignition source. This information can be used to make risk assessments with a higher degree of confidence than estimates made before and it provides valuable information for use in the development of a facility′s Emergency Response Plan.