The heat of combustion is the energy released when a substance burns completely in oxygen. For organic fuels the products are carbon dioxide and water, and the answer depends on what happens to that water. If the water vapour is condensed back to liquid, its latent heat is recovered and you get the higher heating value (HHV, gross calorific value) – the same quantity as the standard enthalpy of combustion ΔHc° listed in chemistry tables. If the water leaves as vapour, as in most engines and furnaces, you get the lower heating value (LHV, net calorific value).
The difference is exactly the heat of vaporization of the water formed: 44.0 kJ per mole at 25 °C (2.44 MJ per kilogram of water). Hydrogen-rich fuels show the biggest gap – the HHV exceeds the LHV by about 18% for hydrogen and 11% for methane – which is why condensing boilers, which recover that heat, can reach “efficiencies” above 100% on an LHV basis.
Per kilogram, hydrogen is the champion at about 142 MJ/kg (HHV), roughly three times petrol, while alcohols carry less energy because they are already partly oxidised. Choose a fuel to see its standard values, or enter a lower heating value and the water formed for any other fuel.
How to use the Heat of Combustion Calculator
- Fuel data
- Fuel
- Lower heating value (LHV)
- Heat of vaporization of water — 2.442 MJ/kg at 25 °C (standard basis); 2.257 MJ/kg at 100 °C.
- Moles of water formed
- Moles of fuel burned
- Molar mass of the fuel
- Mass of fuel burned
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
For listed fuels, the standard enthalpy of combustion (25 °C, liquid water as product) is divided by the molar mass to give the HHV per kilogram, and the vaporization enthalpy of the water formed is subtracted for the LHV. In custom mode the HHV is the LHV plus the latent heat of the water produced per kilogram of fuel. The heat released is the moles burned times the molar enthalpy of combustion.
Worked example
Methane: ΔHc° = −890.3 kJ/mol and M = 16.043 g/mol, so HHV = 890.3 / 16.043 = 55.50 MJ/kg. Burning one mole forms 2 mol of water; subtracting their latent heat, LHV = (890.3 − 2 × 44.0) / 16.043 = 50.01 MJ/kg. Burning 1 kg (62.33 mol) releases 55.5 MJ, or 15.4 kWh, if the water is condensed.
Custom mode reproduces this: LHV = 50.0 MJ/kg, h_fg = 2.442 MJ/kg, 2 mol water per 1 mol fuel, M = 16.043 g/mol: HHV = 50.0 + 2.442 × (2 × 18.015)/(1 × 16.043) = 50.0 + 5.48 = 55.48 MJ/kg.
Standard heats of combustion
| Fuel | ΔHc° (kJ/mol) | HHV (MJ/kg) | LHV (MJ/kg) |
|---|---|---|---|
| Hydrogen, H₂ (g) | −285.8 | 141.77 | 119.94 |
| Methane, CH₄ (g) – natural gas | −890.3 | 55.49 | 50.01 |
| Ethane, C₂H₆ (g) | −1560.7 | 51.90 | 47.51 |
| Propane, C₃H₈ (g) | −2219.2 | 50.33 | 46.34 |
| n-Butane, C₄H₁₀ (g) | −2877.6 | 49.51 | 45.72 |
| n-Pentane, C₅H₁₂ (l) | −3509.0 | 48.64 | 44.98 |
| n-Hexane, C₆H₁₄ (l) | −4163.2 | 48.31 | 44.74 |
| n-Octane, C₈H₁₈ (l) – petrol model | −5470.0 | 47.89 | 44.42 |
| Methanol, CH₃OH (l) | −726.0 | 22.66 | 19.91 |
| Ethanol, C₂H₅OH (l) | −1366.8 | 29.67 | 26.80 |
| Benzene, C₆H₆ (l) | −3267.6 | 41.83 | 40.14 |
| Acetylene, C₂H₂ (g) | −1300.0 | 49.93 | 48.24 |
| Glucose, C₆H₁₂O₆ (s) | −2803.0 | 15.56 | 14.09 |
| Carbon (graphite) | −393.5 | 32.76 | 32.76 |
| Carbon monoxide, CO (g) | −283.0 | 10.10 | 10.10 |
ΔHc° at 25 °C and 1 bar with CO₂(g) and H₂O(l) as products, calculated from NIST/CODATA enthalpies of formation; LHV uses 44.0 kJ/mol for the vaporization of water.
Tips
- Use kelvin for every temperature in thermodynamic formulas – the unit menus convert for you.
- Watch the units: ΔH is usually in kJ/mol and ΔS in J/(mol·K).
- Tabulated values are for standard conditions (1 bar, usually 25 °C).
- Everything is calculated in your browser.
Common mistakes to avoid
- Mixing kJ and J between ΔH and ΔS.
- Assuming ΔH and ΔS do not change with temperature over large ranges.
- Using Celsius in ln(T₂/T₁).
Inputs explained
| Field | What to enter |
|---|---|
| Fuel data | Choose one: Choose a fuel from the list, Custom fuel – convert LHV to HHV |
| Fuel | Choose one: Hydrogen, H₂ (g), Methane, CH₄ (g) – natural gas, Ethane, C₂H₆ (g), Propane, C₃H₈ (g) … |
| Lower heating value (LHV) | Units: MJ/kg, kJ/kg, kJ/g, kcal/g, kWh/kg, BTU/lb |
| Heat of vaporization of water | Units: MJ/kg, kJ/kg, kJ/g, kcal/g, kWh/kg, BTU/lb. 2.442 MJ/kg at 25 °C (standard basis); 2.257 MJ/kg at 100 °C. |
| Moles of water formed | A number |
| Moles of fuel burned | A number |
| Molar mass of the fuel | Units: g/mol, kg/mol, Da (u), kDa |
| Mass of fuel burned | Units: kg, g, lb, t |
What each result means
- Heat of combustion – higher heating value (HHV, gross) = [h]÷[M]
- Molar enthalpy of combustion ΔHc° (HHV basis) = -[h]
- Heat released by the mass burned (HHV) = [Mass of fuel burned]÷[M]×[h]
- Moles of fuel in that mass = [Mass of fuel burned]÷[M]
- Water formed per kg of fuel = [w]×18.015÷[M]
Formula
HHV = LHV + h_fg · (n_water · M_water) / (n_fuel · M_fuel) · HHV (MJ/kg) = |ΔHc°| (kJ/mol) / M (g/mol) · LHV = HHV − n_water × 44.0 kJ/mol ÷ M · Q = m / M × |ΔHc°|Worked example
Inputs — Fuel data: Choose a fuel from the list; Fuel: Methane, CH₄ (g) – natural gas; Mass of fuel burned: 1 kg.
Results — Heat of combustion – higher heating value (HHV, gross) = 55.4946 MJ/kg; Lower heating value (LHV, net) = 50.0093 MJ/kg; Molar enthalpy of combustion ΔHc° (HHV basis) = -890.3 kJ/mol; Latent heat of the water as a share of HHV = 9.88 %; Heat released by the mass burned (HHV) = 55,494.6082 kJ; Heat released by the mass burned (LHV) = 50,009.3499 kJ; Moles of fuel in that mass = 62.3325 mol; Water formed per kg of fuel = 2.2458 kg; CO₂ formed per kg of fuel = 2.7432 kg; CO₂ per unit of heat (HHV) = 49.43 g CO₂/MJ.
Key terms
- Heat of combustion
- The heat released when a unit amount of substance burns completely in oxygen.
- Higher heating value (HHV)
- Heat of combustion with the water produced condensed to liquid; also called gross calorific value.
- Lower heating value (LHV)
- Heat of combustion with the water produced left as vapour; also called net calorific value.
- Standard enthalpy of combustion (ΔHc°)
- The enthalpy change per mole of substance burned under standard conditions (1 bar, 25 °C, liquid water as product).
Frequently asked questions
What is the heat of combustion of methane?
ΔHc° = −890.3 kJ/mol, i.e. a higher heating value of 55.5 MJ/kg; its lower heating value is 50.0 MJ/kg.
What is the difference between HHV and LHV?
HHV counts the heat recovered by condensing the water formed; LHV assumes the water stays as vapour. The gap is 44.0 kJ per mole of water (2.44 MJ/kg of water) at 25 °C.
Which fuel has the highest heat of combustion per kilogram?
Hydrogen: about 141.8 MJ/kg HHV (119.9 MJ/kg LHV), roughly three times that of hydrocarbons, though very little per litre.
How much heat does burning 1 kg of propane give?
About 50.3 MJ (HHV) or 46.3 MJ (LHV) – roughly 14 kWh of heat.
Why is the heat of combustion negative in tables?
Combustion releases heat, so the enthalpy change of the system is negative: ΔHc° = −890.3 kJ/mol for methane. Heating values are quoted as positive numbers.
How much CO₂ does burning a fuel produce?
One mole of CO₂ for every carbon atom: 2.74 kg of CO₂ per kg of methane, 2.99 kg per kg of propane and 3.08 kg per kg of octane.
Why does ethanol release less energy than octane per kilogram?
Ethanol already contains oxygen, so it is partly oxidised: 29.7 MJ/kg versus 47.9 MJ/kg for octane (HHV).