Heat of Combustion Calculator
ChemistryCalculate the heat released during combustion of a fuel from its mass and specific heat of combustion. Convert between kJ, kcal, and kWh for any fuel quantity.
47.9 kJ/g
kJ/g
Heat Released (kJ)
4,790
Heat Released (kcal)
1,144.838
Heat Released (kWh)
1.331
What is a Heat of Combustion?
The Heat of Combustion Calculator computes the total energy released when a given mass of fuel is burned, using the formula: Heat Released (kJ) = Mass (g) ร Specific Heat of Combustion (kJ/g) ร Efficiency (%). It converts the result to kilocalories (kcal) and kilowatt-hours (kWh) simultaneously, covering the three most commonly used energy units in chemistry, food science, and engineering.
Heat of combustion (also called calorific value or specific combustion enthalpy) is the most practical thermodynamic quantity for fuel evaluation. It determines how much energy a given quantity of fuel releases, which directly sets how long a gas cylinder lasts, how much fuel a vehicle requires per kilometre, and how much thermal energy a process plant generates per tonne of coal burned.
For chemical thermodynamics, heat of combustion is also used in Hess's law calculations. Since enthalpy is a state function, the standard enthalpy of formation of a compound can be calculated from measured heats of combustion. This connection to the Gibbs Free Energy Calculator and Entropy Calculator makes calorimetric data foundational to the full thermodynamic characterisation of a substance.
How to use this Heat of Combustion calculator
- Measure or specify the mass of fuel to be burned in grams. Enter it in the Mass of Fuel field.
- Find the specific heat of combustion (calorific value) for your fuel. Common reference values: petrol โ 47.9 kJ/g, LPG โ 49.6 kJ/g, wood โ 14.9 kJ/g, coal โ 29 kJ/g. Enter in the Specific Heat of Combustion field.
- Set the Combustion Efficiency to 100% for theoretical calculations or to the expected practical efficiency (domestic gas burners โ 55โ85%, industrial furnaces โ 85โ95%).
- Read Heat Released (kJ) as the primary energy output. Note the kcal and kWh equivalents for the appropriate application.
- For Hess's law problems, use the heat of combustion values to calculate standard enthalpies of formation using the cycle: ฮH_fยฐ(compound) = ฮฃฮH_comb(elements) โ ฮH_comb(compound).
Formula & Methodology
Core formula:Q = m ร q ร (ฮท / 100)Where: Q = heat released (kJ), m = mass of fuel (g), q = specific heat of combustion (kJ/g), ฮท = efficiency (%) Unit conversions:Q_kcal = Q_kJ / 4.184 Q_kWh = Q_kJ / 3600Worked example โ LPG cylinder energy content: A standard 14.2 kg LPG cylinder, specific heat of combustion = 49.6 kJ/g, household burner efficiency = 68%.Q = 14,200 g ร 49.6 kJ/g ร (68 / 100) = 14,200 ร 49.6 ร 0.68 = 478,777 kJ โ 478.8 MJ of useful heat Q_kcal = 478,777 / 4.184 = 114,415 kcal Q_kWh = 478,777 / 3,600 = 133.0 kWhAt current LPG prices of approximately โน900 per cylinder, this equals โน6.77 per kWh of useful heat delivered โ significantly more expensive than grid electricity at โน5โ8/kWh in most Indian cities but offering greater portability and no infrastructure dependency in rural areas.
Frequently Asked Questions
Heat of combustion (also called enthalpy of combustion or calorific value) is the amount of heat energy released when a unit quantity of a substance is completely burned in excess oxygen at standard conditions. It is expressed as kJ/g, kJ/mol, or kcal/g. The heat of combustion is always negative (exothermic) for fuel combustion, but the magnitude โ the calorific value โ is quoted as a positive number. Higher calorific value means more energy per gram of fuel.
Heat Released (kJ) = Mass of Fuel (g) ร Specific Heat of Combustion (kJ/g) ร (Efficiency / 100). The specific heat of combustion is the energy released per gram of fuel at complete combustion. Common values: hydrogen โ 141.8 kJ/g, natural gas (methane) โ 55.5 kJ/g, LPG โ 49.6 kJ/g, octane โ 47.9 kJ/g, ethanol โ 29.7 kJ/g, wood โ 14.9 kJ/g, coal โ 24โ35 kJ/g.
Higher calorific value (HCV, also called gross calorific value) includes the heat recovered from condensing the water vapour produced during combustion. Lower calorific value (LCV, also called net calorific value) does not include condensation heat โ it assumes water leaves as vapour. LCV is typically 5โ10% lower than HCV. Practical combustion systems (boilers, engines) rarely recover condensation heat, so LCV is used for efficiency calculations. For octane (petrol), HCV โ 47.9 kJ/g and LCV โ 44.4 kJ/g.
Key specific combustion values: Hydrogen: 141.8 kJ/g (highest by mass), LPG: 49.6 kJ/g, Petrol (octane): 47.9 kJ/g, Diesel: 44.8 kJ/g, Natural gas (methane): 55.5 kJ/g, Kerosene: 46.2 kJ/g, Ethanol: 29.7 kJ/g, Coal (bituminous): 24โ35 kJ/g, Wood: 14โ19 kJ/g, Charcoal: 30 kJ/g. In India, LPG (44.5 MJ/kg) and CNG (50 MJ/kg) are the dominant domestic and vehicular fuels, with heat content being a key specification in BIS standards.
Enter the mass of fuel in grams in the 'Mass of Fuel' field. Enter the specific heat of combustion (calorific value) of your fuel in kJ/g in the 'Specific Heat of Combustion' field. Enter the combustion efficiency as a percentage โ use 100% for theoretical complete combustion or a realistic efficiency (e.g., 85% for a typical gas burner). The calculator returns heat released in kJ, kcal, and kWh.
1 kcal = 4.184 kJ (the thermochemical definition). 1 kWh = 3,600 kJ. So to convert: kJ โ kcal: divide by 4.184; kJ โ kWh: divide by 3,600. These conversions matter in comparing fuel energy content across different unit systems โ dietary energy labels in India use kcal, electricity bills use kWh, and engineering calculations use kJ. The calculator returns all three simultaneously.
Combustion efficiency is the fraction of the fuel's chemical energy that is actually converted to useful heat. Incomplete combustion (insufficient oxygen) leaves unburned fuel (carbon monoxide, soot) that represents lost energy. Real combustion systems achieve 70โ95% efficiency depending on design: domestic gas burners โ 55โ85%, industrial furnaces โ 80โ90%, diesel engines โ 35โ45%, petrol engines โ 25โ35%. A 1 kg LPG cylinder releasing 49,600 kJ theoretical at 85% efficiency delivers 42,160 kJ of useful heat.
Heat of combustion is measured using a bomb calorimeter โ a sealed, high-pressure vessel filled with oxygen in which a measured mass of fuel is burned electrically. The heat released raises the temperature of a surrounding water bath, and knowing the heat capacity of the system gives ฮH_comb = โC_cal ร ฮT / n, where C_cal is the calorimeter constant, ฮT is the temperature rise, and n is the moles of fuel. Bomb calorimetry measures constant-volume heat (ฮU); the constant-pressure value (ฮH) differs by a small gas-phase work correction: ฮH = ฮU + ฮnRT.
Very much so. India's domestic cooking energy comes primarily from LPG (Pradhan Mantri Ujjwala Yojana has distributed over 100 million LPG connections), piped natural gas (city gas distribution), and biomass. Knowing the heat of combustion per unit mass and comparing it to per-kg prices allows consumers and policy planners to calculate cost per unit of useful energy. LPG at โน900 per 14.2 kg cylinder (49.6 kJ/g, 85% efficiency) costs roughly โน1.35 per MJ of useful heat โ a reference for comparing PNG tariffs and electricity.
The heat of combustion of glucose (CโHโโOโ) is about โ2,803 kJ/mol or โ15.6 kJ/g. This is the maximum energy theoretically available from complete oxidation: CโHโโOโ + 6Oโ โ 6COโ + 6HโO. Cells capture approximately 38 ATP per glucose through aerobic respiration, recovering about 40% of this energy as chemical energy in ATP (~30.5 kJ/mol ATP ร 38 = 1,159 kJ captured), with the rest released as heat. The metabolic efficiency of โ40% compares favourably with heat engines; it explains why resting metabolism generates significant body heat.