This paper suggests a method of estimating the dimensions of gas jet fires in the buoyancy-dominated regime. The results should be of use to hazard analysts concerned with radiation and flame impingement. The length of a jet fire is determined by modifying the large-scale experimental data for propane of Hustad and Sonju (Progr. Astronaut. Aeronaut. 1985, 95) using a stoichiometric factor based on the method of Hawthorne et al. ('Third Symp. Combus. Flame Explosion Phenomena', 1949, p. 266). Agreement with the experimental results of Hustad and Sonju for methane is excellent. The jet fires are assumed to be cylindrical and the lateral dimension is estimated using the method offered by Marshall (IChemE Symposium Series No. 49, 1977, p. 99), based on momentum and dispersion considerations. The lateral dimension is over-estimated by comparison with experiment, but radiation calculations compare well. Copyright (C) 1996 Elsevier Science Ltd