What is the heat of combustion?

The heat of combustion (ΔH°comb) is the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions (298 K, 1 atm). It is always negative (exothermic) for real fuels, indicating that heat is released.

Why is the heat of combustion always negative?

Combustion releases energy — the chemical bonds in the products (CO₂ and H₂O) are more stable than those in the reactants (fuel and O₂). By convention a negative ΔH means the system loses energy to the surroundings as heat.

What is the difference between higher and lower heating value?

The higher heating value (HHV) assumes water produced is liquid (H₂O(l)); the lower heating value (LHV) assumes it is gaseous (H₂O(g)). This tool uses the HHV convention with H₂O(l), giving slightly larger (more negative) values than LHV tables.

How does molar mass affect kJ/g results?

The kJ/g value is simply ΔH°comb (kJ/mol) divided by the molar mass (g/mol). Lighter fuels like hydrogen have a very high energy per gram despite a modest per-mole value, because their molar mass is tiny.

What standard enthalpy of formation values does this tool use?

ΔH°f for CO₂(g) = −393.5 kJ/mol and H₂O(l) = −285.8 kJ/mol, taken from NIST WebBook standard reference data. Preset fuel values are also NIST-sourced standard enthalpies of formation at 298 K.

Can I use this for compounds with nitrogen or sulfur?

The custom mode handles CₓHᵧOᵤ compounds. For fuels containing N or S the balanced equation and product set change (N₂, SO₂ form), which is outside this tool's scope. For those, use the general heat-of-reaction calculator.

Heat of Combustion Calculator

Name: Heat of Combustion Calculator
Availability: InStock
Author: Nham Vu

Select a fuel or enter custom enthalpy values to instantly calculate the heat of combustion in kJ/mol, kJ/g, and MJ/kg.

Fuel / Compound

Select a preset fuel

Select a fuel and click Calculate to see results.

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Summary

Select a fuel or enter custom enthalpy values to instantly calculate the heat of combustion in kJ/mol, kJ/g, and MJ/kg.

How it works

Choose a preset fuel from the dropdown, or select "Custom compound" to enter your own.
For a custom compound, provide the molecular formula (e.g. C3H8) and its standard enthalpy of formation in kJ/mol.
The calculator writes the balanced combustion equation and looks up ΔH°f for CO₂(g) and H₂O(l).
It applies ΔH°comb = Σ ΔH°f(products) − Σ ΔH°f(reactants) to compute the molar heat of combustion.
Results are displayed in kJ/mol, kJ/g (using molar mass), and MJ/kg for easy comparison across fuels.
Copy any result to the clipboard with one click.

Use cases

Compare the energy density of different fuels (methane vs. propane vs. gasoline).
Solve thermochemistry problems in general or organic chemistry courses.
Estimate heat output for combustion reactions in engineering calculations.
Verify textbook values for standard heats of combustion.
Calculate fuel requirements for a given energy output target.
Cross-check calorimetry experiment results against theoretical values.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu