The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it.

The calorific value is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon or other organic molecule reacting with oxygen to form carbon dioxide and water and release heat. It may be expressed with the quantities:

There are two kinds of enthalpy of combustion, called higher and lower heating value, depending on how much the products are allowed to cool and whether compounds like H
2O are allowed to condense.The high heat values are conventionally measured with a bomb calorimeter. Low heat values are calculated from high heat value test data. They may also be calculated as the difference between the heat of formation ΔH⦵
f of the products and reactants (though this approach is somewhat artificial since most heats of formation are typically calculated from measured heats of combustion). For a fuel of composition CcHhOoNn, the (higher) heat of combustion is 418 kJ/mol (c + 0.3 h − 0.5 o) usually to a good approximation (±3%), though it can be significantly off if o + n > c (for instance in the case of nitroglycerine, C
3H
5N
3O
9, this formula would predict a heat of combustion of 0). The value corresponds to an exothermic reaction (a negative change in enthalpy) because the double bond in molecular oxygen is much weaker than other double bonds or pairs of single bonds, particularly those in the combustion products carbon dioxide and water; conversion of the weak bonds in oxygen to the stronger bonds in carbon dioxide and water releases energy as heat.

By convention, the (higher) heat of combustion is defined to be the heat released for the complete combustion of a compound in its standard state to form stable products in their standard states: hydrogen is converted to water (in its liquid state), carbon is converted to carbon dioxide gas, and nitrogen is converted to nitrogen gas. That is, the heat of combustion, ΔH°comb, is the heat of reaction of the following process: