Combustion Reaction Calculator
The oxygen and air a hydrocarbon needs to burn, and the carbon dioxide and water it produces — from the formula CₓHᵧ.
Complete combustion of CₓHᵧ needs x + y/4 moles of O₂ and produces x moles of CO₂ and y/2 of H₂O.
How the combustion reaction calculator works
Complete combustion of CₓHᵧ needs x + y/4 moles of O₂ and produces x moles of CO₂ and y/2 of H₂O. Air is 21% oxygen by mole, so the air requirement is 4.76 times the oxygen.
That ratio explains why burning fuel produces so much more CO₂ than the fuel weighed: a kilogram of petrol makes about 3.1 kg of carbon dioxide, because most of the mass comes from the oxygen it took out of the air.
Formula: CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O
Worked examples
| Inputs | CO₂ produced | Note |
|---|---|---|
| Octane (petrol), 1 kg | 3.0821 kg | 3.08 kg of CO₂ |
| Methane (natural gas) | 2.7432 kg | 2.74 kg — the lowest of any hydrocarbon |
| Carbon alone | 3.6641 kg | 3.66 kg — no hydrogen to take up oxygen |
FAQFrequently asked questions
Why does a kilogram of petrol make three kilograms of CO₂?
Because most of the carbon dioxide's mass is oxygen taken from the air. Carbon is 12; carbon dioxide is 44.
Which fuel makes the least CO₂ per kilogram?
Methane, at 2.74 kg — it has the highest hydrogen-to-carbon ratio of any hydrocarbon. Per unit of energy the advantage is bigger still.
How much air does burning need?
About 15 kg of air per kilogram of petrol — roughly 12 cubic meters, which is why engines are essentially air pumps.
Does this assume complete combustion?
Yes. Incomplete combustion makes carbon monoxide and soot instead, producing less CO₂ but far worse pollution.
What about the water?
A kilogram of petrol makes about 1.4 kg of water vapour. It is the visible plume from a cold exhaust on a winter morning.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.