Heat of Combustion Calculator
The energy released by burning a given mass of a fuel — in kilojoules, megajoules and kilowatt-hours — from its heat of combustion, with the energy per gram and the carbon dioxide produced.
The heat of combustion is the enthalpy released when one mole of a substance burns completely in oxygen.
How the heat of combustion calculator works
The heat of combustion is the enthalpy released when one mole of a substance burns completely in oxygen. Divide the mass by the molar mass for moles, multiply by ΔH_c for the energy. Per gram, hydrogen is the richest fuel and hydrocarbons cluster near 45–50 kJ/g; alcohols and sugars carry oxygen already and give less. Each carbon atom leaves as one CO₂.
Formula: energy = (mass ÷ M) × ΔH_c; CO₂ = moles × carbons × 44.01 g
Worked examples
| Inputs | Energy released | Note |
|---|---|---|
| 100 g of methane | 5,550.5 kJ | 5,550 kJ; 274 g CO₂ |
| 1 kg of petrol (octane) | 47,885.8 kJ | 47,886 kJ; 3.08 kg CO₂ |
| 10 g of hydrogen | 1,417.7 kJ | 1,418 kJ; no CO₂ |
FAQFrequently asked questions
Higher or lower heating value?
These are standard enthalpies of combustion with water as liquid — the higher (gross) heating value. Engines and boilers exhaust water as vapour, so they realize the lower value, about 10% less for hydrogen-rich fuels.
Why is hydrogen so high per gram but low per mole?
Its molar mass is tiny: 285.8 kJ per 2 g is 142 kJ/g, three times petrol. Per liter it is the reverse, because hydrogen gas is so light — the storage problem of hydrogen fuel.
Does this apply to food?
Yes — glucose at 15.6 kJ/g is the 3.7 kcal/g of carbohydrate on a label, and fats burn at about 39 kJ/g. Nutrition labels use slightly lower "metabolisable" values.
Where does the CO₂ figure come from?
Complete combustion turns every carbon atom into one CO₂ molecule at 44.01 g/mol. Petrol (octane) makes about 3.1 kg of CO₂ per kilogram burned — the number behind vehicle emissions factors.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.