While chemistry provides methods of calculating the heating value of a certain amount of a substance, there is a difference between this theoretical value and its application to real coal. The grade of a sample of coal does not precisely define its chemical composition, so calculating the actual usefulness of coal as a fuel requires determining its proximate and ultimate analysis (see "Chemical Composition" below).
The calorific value Q of coal is the heat liberated by its complete combustion with oxygen. Q is a complex function of the elemental composition of the coal. Q can be determined experimentally using calorimeters. Dulong suggests the following approximate formula for Q when the oxygen content is less than 10%:
where C is the mass percent of carbon, H is the mass percent of hydrogen, O is the mass percent of oxygen, and S is the mass percent of sulfur in the coal. With these constants, Q is given in kilojoules per kilogram.