The heats of formation (HOF) for 49 tetrazole derivatives are calculated with the density functional theory (DFT) B3LYP method by means of designed isodemic and isogyric reactions. The average absolute deviation for five compounds for which the experimental HOFs are available is less than 2 kcal/mol target accuracy of G-2 theory. It has been demonstrated that for compounds involving delocalized bends, choosing molecules containing all of the delocalized bonds as reference compounds is an appropriate approach. The calculated HOFs indicate that most neutral 2H-isomers are more stable than the corresponding 1H-isomers whereas the 1-substituted tetrazolate anions are more stable than the corresponding 2-substituted ones. Furthermore, our results consistently show that C-substituted tetrazoles are more stable than the corresponding N-substituted isomers. Our calculated heat of formation calls into question the experimental heat of formation of 2-methyltetrazole.