The peak gas pressure developed as a result of a confined explosion is an important parameter characterizing the pressure signal and is crucial for assessment of the structural response of the confined space envelope elements. The gas pressure depends on the amount of the released energy, including the afterburning energy. A new analytical model is presented to predict the gas pressure developed in a confined volume, for different types of explosives, as function of the charge weight (W) and the free confined volume (V), considering the afterburning effect. The model is based on the detonation chemical reaction and on the calculation of a full or partial afterburning energy release due to the reaction of the fuels in the detonation products with the surrounding oxygen. Considering the detonation energy and the possible additional afterburning energy, the model demonstrates the different behavior of the gas pressure variation with W/V for different types of explosives. The equivalent TNT charge weight is calculated for each explosive. According to the analysis performed, the TNT equivalent factor varies with W/V. The predictions of the model results are compared with available models as well as with available test data and very good agreement is obtained.