A novel method is presented to estimate the prediction of the specific impulse of mostly ideal and less ideal compositions of explosives. It requires only elemental compositions and some structural parameters of pure and composite explosives. There is no need to use the heat of formation of the explosive, which is an essential input parameter to compute the specific impulse through complicated computer codes. Moreover, there is no need to use any assumed decomposition products to calculate the specific impulse. The new simple method gives good results for any CaHbNcOd explosives, even with complicated molecular structures, and their composite mixtures. The calculated specific impulse can also be used to estimate detonation pressures and velocities of pure and mixed explosives at specified loading densities.