The heat of formation (ΔHf) is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states. It is a measure of the energy released or absorbed during the formation of a compound.

The heat of formation is closely related to enthalpy changes and Hess's Law. According to Hess's Law, the enthalpy change of a reaction can be calculated by considering the enthalpy changes of other reactions.