The formation reactions for most organic compounds are hypothetical. For instance, carbon and hydrogen will not directly react to form methane (), so that the standard enthalpy of formation cannot be measured directly. However the standard enthalpy of combustion is readily measurable using bomb calorimetry. The standard enthalpy of formation is then determined using Hess's law. The combustion of methane:
is equivalent to the sum of the hypothetical decomposition into elements followed by the combustion of the elements to form carbon dioxide () and water ():
Applying Hess's law, 
Solving for the standard of enthalpy of formation,
The value of  is determined to be −74.8 kJ/mol. The negative sign shows that the reaction, if it were to proceed, would be exothermic; that is, methane is enthalpically more stable than hydrogen gas and carbon.
It is possible to predict heats of formation for simple unstrained organic compounds with the heat of formation group additivity method.