Chemistry

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Heat of Combustion Calculator.

Calculate gross and useful combustion energy from mass, explicit specific energy, and efficiency.

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Results update as you type.

Gross energy: 40 MJ

Gross energy

40.00MJ
Useful energy: 40 MJ

Useful energy

40.00MJ
Useful energy: 11.11 kWh

Useful energy

11.11kWh

How to Use

Gross energy = mass × specific energy; useful energy = gross energy × efficiency. Fuel data is never looked up.

Use a named source for any material-specific coefficient. This page does not infer local rates, species tables, laboratory methods, or safety guidance.

Use Cases

Estimate energy from fuel combustion

Quickly determine the total heat energy available from a given mass of fuel using its specific energy value.

Example: 10 kg of wood with specific energy 15 MJ/kg yields 150 MJ gross.

Assess usable energy after losses

Incorporate conversion efficiency to see how much energy is actually useful for heating or power generation.

Example: At 70% efficiency, 150 MJ gross becomes 105 MJ useful.

Frequently Asked Questions

How do I calculate the gross heat of combustion?
Multiply the fuel mass (kg) by the specific energy (MJ/kg). The result is the gross energy released during complete combustion, in megajoules (MJ).
What is the useful energy output?
Useful energy is the gross energy multiplied by the conversion efficiency (as a decimal). For example, 100 MJ gross at 80% efficiency gives 80 MJ useful energy.
What units are used in this calculator?
Fuel mass is in kilograms (kg), specific energy in megajoules per kilogram (MJ/kg), and efficiency in percent (%). The results are in megajoules (MJ).

Tips & Common Mistakes

Tips

  • Ensure specific energy is in MJ/kg; if you have it in kJ/kg, divide by 1000 first.
  • Efficiency should be entered as a percentage (e.g., 85 for 85%).
  • Use consistent units: mass in kg, specific energy in MJ/kg for correct results.
  • Remember that gross energy assumes complete combustion; real-world losses may reduce it.

Common Mistakes to Avoid

  • Entering efficiency as a decimal (0.8) instead of a percentage (80).
  • Using specific energy in different units without converting to MJ/kg.
  • Forgetting to multiply by efficiency when calculating useful energy.

Last updated: August 13, 2026