Combustion Heat Calculator

A combustion heat calculator finds the energy released when a fuel burns completely in oxygen. It converts an amount of substance, in moles, grams, or kilograms, into an energy value in kilojoules, megajoules, or kilocalories.

What is heat of combustion?

Heat of combustion, also called enthalpy of combustion, is the energy released when one mole of a substance burns completely in oxygen under standard conditions (25°C and 1 atmosphere of pressure). Burning is an exothermic reaction, meaning it releases energy rather than absorbing it. The value is usually written in kilojoules per mole (kJ/mol).

Every substance has its own fixed heat of combustion, found by experiment. A calculator does not measure this value. It looks up a known value and scales it to the amount of fuel entered.

The combustion reaction

Most fuels used in this calculator are hydrocarbons or related organic compounds. When they burn completely, they react with oxygen to form carbon dioxide and water, and release heat:

Fuel + O₂ → CO₂ + H₂O + heat

For example, methane burns as CH₄ + 2O₂ → CO₂ + 2H₂O + heat.

How to calculate heat of combustion

The total energy released equals the amount of substance, in moles, multiplied by its heat of combustion per mole:

Total heat = moles × heat of combustion (kJ/mol)

If the amount is given in grams or kilograms instead of moles, convert it first using the substance's molar mass, the mass of one mole in grams:

Moles = mass in grams ÷ molar mass (g/mol)
Moles = (mass in kilograms × 1000) ÷ molar mass (g/mol)

Once the energy is calculated in kilojoules, it can be converted to other units:

1 MJ = 1,000 kJ
1 kJ ≈ 0.239 kcal
1 kcal ≈ 4.184 kJ

Worked example

How much heat comes from burning 10 grams of methane (CH₄)?

Molar mass of CH₄: 16.04 g/mol
Convert to moles: 10 g ÷ 16.04 g/mol ≈ 0.623 mol
Heat of combustion of methane: 890 kJ/mol
Total heat: 0.623 mol × 890 kJ/mol ≈ 555 kJ

That amount of heat also equals about 0.555 MJ or 133 kcal.

Heat of combustion values for common substances

The table lists the standard heat of combustion and molar mass for each substance in the calculator, along with the energy released per gram.

Substance	Formula	Heat of combustion (kJ/mol)	Molar mass (g/mol)	Energy per gram (kJ/g)
Hydrogen	H₂	286	2.02	141.6
Methane	CH₄	890	16.04	55.5
Acetylene	C₂H₂	1,300	26.04	49.9
Ethane	C₂H₆	1,560	30.07	51.9
Methanol	CH₃OH	726	32.04	22.7
Propane	C₃H₈	2,220	44.10	50.3
Ethanol	C₂H₅OH	1,367	46.07	29.7
Carbon monoxide	CO	283	28.01	10.1
Butane	C₄H₁₀	2,877	58.12	49.5
Glucose	C₆H₁₂O₆	2,805	180.16	15.6

Per mole, butane releases the most energy of the substances listed, followed by glucose. Per gram, hydrogen releases far more energy than any of the others, because its molar mass is so low.

Why energy density varies

Molecules built mostly from carbon and hydrogen release energy from breaking C–H and C–C bonds and forming new bonds in CO₂ and H₂O. Larger hydrocarbons generally release more energy per mole, because they have more bonds. But energy per gram depends on molar mass too, so a light molecule like hydrogen can release more energy per gram even though it releases less per mole.

Substances that already contain oxygen, like ethanol and glucose, release less energy per gram than pure hydrocarbons of similar size. Some of their bonds are already partly oxidized before combustion starts, so less energy is available to release.

How heat of combustion is measured

The standard method is bomb calorimetry. A weighed sample burns inside a sealed, oxygen-filled steel container submerged in a known mass of water. The heat released warms the water, and the temperature rise, combined with the calorimeter's known heat capacity, gives the energy released.

Higher and lower heating value

Heat of combustion can be reported two ways, depending on whether the water produced ends up as liquid or vapor. The higher heating value (HHV) counts the extra heat released when water vapor condenses to liquid. The lower heating value (LHV) does not. The values used in this calculator are HHV figures, measured with the water product as liquid.

Frequently asked questions

What is the difference between higher and lower heating value? Higher heating value (HHV) includes the heat released when water vapor in the combustion products condenses to liquid water. Lower heating value (LHV) leaves that water as vapor, so it reports a smaller number. This calculator uses HHV values.

Which fuel has the highest heat of combustion? Among the substances in this calculator, butane releases the most energy per mole (2,877 kJ/mol). Per gram, hydrogen releases the most (about 141.6 kJ/g), because its molar mass is very low.

Can I calculate heat of combustion for a substance not listed? This calculator only covers the ten substances it includes. For any other compound, its heat of combustion must come from a chemistry reference or laboratory measurement, then be entered into the same formula by hand.

Why do carbohydrates like glucose release less energy per gram than hydrocarbons? Glucose already contains oxygen atoms in its structure, so its carbon atoms are partly oxidized before combustion begins. Less energy is left to release compared with a pure hydrocarbon of similar size.

Is heat of combustion the same as calories in food? They measure the same kind of quantity, energy released by oxidizing a substance, but food Calories (kilocalories) reported on labels come from a mix of nutrients and account for what the body actually absorbs and metabolizes, not just the maximum chemical energy from full combustion.

Are combustion reactions dangerous to test at home? Combustion is exothermic and can ignite surrounding material or build up pressure in enclosed spaces. Testing fuels or building calorimeters should only be done with proper training, ventilation, and safety equipment.

Combustion Heat Calculator | Heat of Combustion