Flammability limits are two of the most important variables used to assess the fire and explosion hazards of gases and vapors, and inerting is frequently used in industry to reduce fire and explosion hazards. A model based on the energy balance equation that takes into account thermal radiation loss and constant flame temperature was developed to estimate the flammability envelope of fuel-air-diluent mixtures tested in a constant pressure vessel. The validation of the model was done by comparing the estimated values with the corresponding experimental data for methane, ethylene, propane, propylene, isobutane, and methyl formate with nitrogen or carbon dioxide as the inert gas. The difference in the estimated flammability envelopes between the different heat loss considerations is small. The predictions of the lower flammability limits are in excellent agreement with the experimental data, except for the region approaching the limiting oxygen concentration, where the assumption of complete consumption of the fuel fails. The different behaviors of the lower flammability limit variations for nitrogen and carbon dioxide, when these are both used as inert gases, are attributable to different trends in heat capacity values between air and the two inert gases. Because of the ambiguity of the combustion product distributions, the estimated upper flammability limits are not as precise as the estimated lower flammability limits; however, they are acceptable. Overall, the model describes the experimental data well.