The McClelland formula, based on the upper bound $$\sqrt{2mn}$$ 2 m n , is capable of reproducing over 99.5% of the total $$\pi $$ π -electron energy ( $$E_\pi $$ E π ) of a conjugated hydrocarbon, whose molecules possess n carbon atoms and m carbon–carbon bonds. Its weak point is that it predicts equal $$E_\pi $$ E π -values for all isomers. We now show how this failure can be overcome, offering a general strategy for extending McClelland’s formula. By means of one of these extensions, $$E_\pi $$ E π is related with the energy of the highest occupied molecular orbital, and the error of the new formula is diminished by more than $$50\%$$ 50 % relative to the standard McClelland approximation.