The difficulty in measuring the heats of combustion of polychlorinated dibenzofurans (PCDFs) has resulted in a shortage of data on their heats of formation, required for the purpose of developing an understanding of the role of thermodynamics and kinetics in their production via industrial processes. B3LYP density functional theory calculations have been carried out on a number of PCDFs using 6-31G(d) and 6-311+G(3df,p) basis sets to estimate their heats of formation based on the known experimental values for dibenzofuran, benzene and chlorobenzene. By examining the interactions among chlorine substituents, it is shown that energy contributions arising from successive chlorination can be interpreted in a predictable way, based on a small number of key energy parameters associated with ring position and chlorine atom repulsions. These parameters have been presented as the basis for a simplified prediction algorithm, which can be used to reproduce the predicted DFT heat of formation to within a few kJ/mol, avoiding the need to carry out extensive DFT calculations on the possible 135 isomers of the dibenzofuran group.