Heat of Combustion Calculator — Fuel Energy Release
Enter the fuel type and mass to find the total heat released on complete combustion. The calculator uses published lower heating values (LHV) and converts the result to MJ, kWh and BTU. A comparison bar shows how the chosen fuel ranks against common alternatives for the same mass.
Fuel type
kg
Total energy from complete combustion at lower heating value (water stays as vapour)
- 1
Lower heating value (LHV)
50Energy released per kilogram of fuel on complete combustion (water stays as vapour). - 2
Heat released (Q = m × LHV)
1 × 50 = 50
How does this calculator work?
Heat released Q = m × LHV, where m is fuel mass (kg) and LHV is the lower heating value (MJ/kg). Standard values: natural gas 50, hydrogen 120, petrol 43.5, diesel 43, coal 29, wood 17 MJ/kg. Convert MJ to kWh by × 0.2778 or to BTU by × 947.8.
Formula
How this is calculated
When a fuel burns completely, it releases energy equal to the product of the mass burned and the fuel's specific heat of combustion — also called the calorific value or heating value. The lower heating value (LHV, or net calorific value) is the heat released when combustion products are exhausted as vapour, without recovering the latent heat of the water formed. The higher heating value (HHV, or gross calorific value) includes that latent heat but is harder to exploit in practice; most engineering energy calculations use LHV. Values used here (2024 reference): natural gas 50.0 MJ/kg, hydrogen 120.0 MJ/kg, LPG 46.4 MJ/kg, petrol 43.5 MJ/kg, diesel 43.0 MJ/kg, ethanol 26.8 MJ/kg, coal 29.0 MJ/kg, dry wood 17.0 MJ/kg.
The formula Q = m × LHV is simply mass times the energy density per unit mass. Note that LHV values vary by fuel composition (e.g., coal rank, wood moisture content) and by the exact isomer mixture in petroleum products; the values here are standard reference figures for typical compositions. For volumetric fuels (petrol, diesel, gas in m³), you need the density to convert volume to mass first.
Conversions: 1 MJ = 0.277 778 kWh = 947.817 BTU. These are exact by definition, making unit switching straightforward once the total MJ is known.
Frequently asked questions
The lower heating value (LHV) assumes combustion products leave as water vapour; the higher heating value (HHV) also recovers the latent heat of condensation of that water, giving a higher number. For most combustion engines and gas appliances, LHV is the standard used to rate efficiency. Condensing boilers can partially recover latent heat, so they quote efficiency relative to the lower value, sometimes exceeding 100% on an LHV basis.
Hydrogen's LHV (120 MJ/kg) is about 2.7 times that of natural gas by mass. This is because the hydrogen–oxygen bond releases more energy per kilogram than carbon–hydrogen bonds in hydrocarbons. The drawback is hydrogen's very low density (0.082 kg/m³ at STP), so by volume it stores less energy than natural gas, requiring high-pressure tanks or liquid storage.
Multiply the LHV (MJ/kg) by the fuel density. For example, petrol density ≈ 0.74 kg/L gives an LHV of 43.5 × 0.74 ≈ 32.2 MJ/L; diesel at 0.83 kg/L gives 43.0 × 0.83 ≈ 35.7 MJ/L. This "volumetric energy density" explains why diesel-engined vehicles often achieve better range per litre.
Also known as
TG we-Calculate Editorial Team. (2026). Heat of Combustion Calculator — Fuel Energy Release [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/heat-of-combustion-calculator
TG we-Calculate Editorial Team. "Heat of Combustion Calculator — Fuel Energy Release." TG we-Calculate. 2026. https://we-calculate.com/calculator/heat-of-combustion-calculator.
TG we-Calculate Editorial Team, "Heat of Combustion Calculator — Fuel Energy Release," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/heat-of-combustion-calculator
@misc{wecalculate_heat_of_combustion_calculator, title = {Heat of Combustion Calculator — Fuel Energy Release}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/heat-of-combustion-calculator}}, year = {2026}, note = {TG we-Calculate} }
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