Heat of Combustion Calculator

Background.

Burning a fuel rearranges its atoms into lower-energy products — carbon dioxide and water for anything built from carbon and hydrogen — and the energy difference comes out as heat. How much heat each kilogram gives up is a property of the fuel called its heat of combustion, or calorific value. Methane, the main component of natural gas, releases about 55.5 MJ per kilogram on the gross basis; hydrogen releases about 142. Multiply the per-kilogram value by the mass burned and you have the total: Q = m × H_c. The multiplication is trivial. Choosing the right H_c is not.

The first trap is that every fuel has two heating values. Hydrogen in a fuel burns to water, and whether that water ends up as liquid or vapour changes the answer. The higher heating value (HHV, or gross calorific value) counts the latent heat released when the vapour condenses; the lower heating value (LHV, or net) assumes it leaves as steam. For methane the gap is about 10%, 55.5 against 50.0 MJ/kg, and efficiency figures are quoted on either basis, so mixing them quietly skews any comparison.

The second is units. Tables list heats of combustion in MJ/kg, kJ/g (numerically the same), kJ/mol, BTU/lb and kcal/g, while the field here expects joules per kilogram, so 55.5 MJ/kg goes in as 55,500,000.

The third is sign. Thermochemistry writes an enthalpy of combustion as negative, because the burning system loses energy; methane's is about −890 kJ/mol. Here both inputs are positive magnitudes and the answer is the size of the heat released.

That answer assumes complete combustion. How much of it actually reaches the water in a boiler or the pistons of an engine is a separate question of efficiency.

What is heat of combustion calculator?

The heat of combustion of a substance is the heat released when a unit amount of it burns completely in oxygen, with the products returned to a reference temperature, usually 25 °C. Per kilogram, it is the fuel's specific energy or calorific value. It comes in two versions: gross, or higher heating value, with the water formed condensed to liquid, and net, or lower heating value, with that water left as vapour. The heat released by a given mass is the per-kilogram value times the mass, in joules.

How to use this calculator.

1. 01Enter the Fuel mass in kilograms. A litre of liquid fuel is not a kilogram, so convert volumes with the fuel's density first.
2. 02Enter the Heat of combustion magnitude in joules per kilogram, as a positive number. Multiply MJ/kg or kJ/g by 1,000,000, so methane's 55.5 MJ/kg becomes 55,500,000 J/kg.
3. 03Pick the basis before you pick the number: the gross value if flue gases are cooled enough to condense their water, as in a condensing boiler; the net value if the water leaves as vapour.
4. 04Read the released heat in joules. Divide by 1,000,000 for megajoules or by 3,600,000 for kilowatt-hours.

The formula.

|Q| = m × H_c. Here m is the mass of fuel burned in kilograms, H_c is the magnitude of the heat of combustion per kilogram in joules per kilogram, and |Q| is the magnitude of the heat released in joules; the kilograms cancel, leaving joules. The relationship is strictly proportional: twice the fuel gives twice the heat, and a fuel with twice the heating value gives twice the heat. There is no fuel library and no built-in constant, so the value you enter for H_c carries all of the chemistry. The absolute-value bars appear because thermodynamics gives combustion a negative enthalpy change; the result reports how much heat leaves the fuel, not its sign. Completeness of burning, the gross-or-net choice and equipment efficiency all sit outside the equation and must be built into H_c or applied afterward. The result is given to twelve significant figures.

A worked example.

Frequently asked questions.

References& sources.

1. [1]ASTM D240-19, Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (BIBLIOGRAPHIC/PAYWALL).
2. [2]ISO 1928:2020, Solid mineral fuels — Determination of gross calorific value (BIBLIOGRAPHIC/PAYWALL).
3. [3]BIPM, The International System of Units (SI Brochure), 9th ed., version 3.01, coherent derived units and quantity equations.