Why One Kilogram of Jet Fuel Makes Three of CO₂

Burn a kilogram of jet fuel and you get about 3.16 kilograms of carbon dioxide. The number surprises people because it is larger than the fuel — and the explanation is the most useful thing to understand about aviation emissions.

The mass comes from the air

Kerosene is mostly carbon and hydrogen. Combustion combines each carbon atom with two oxygen atoms drawn from the atmosphere, and oxygen is heavier than carbon — 16 against 12 for each atom, so a CO₂ molecule weighs 44 against the carbon's 12, nearly four times as much.

Not every kilogram of fuel is carbon, so the ratio lands a little below that, at about 3.16. The jet fuel CO₂ calculator uses that as its default because it is the standard aviation emission factor.

The factor is fixed; the fuel burn is not

This matters for how to think about efficiency. Nothing an engine does changes how much CO₂ comes out per kilogram burned — that is chemistry, not engineering. Every efficiency gain has to come from burning less fuel.

So an airline's emissions are, to a very good approximation, its fuel bill in a different unit. Aerodynamics, engine efficiency, weight reduction, routing and load factors all work by reducing fuel; none of them changes the 3.16.

A tonne of fuel is 3.16 tonnes of CO₂

The arithmetic scales directly, which makes flight-level figures easy to reach. A narrowbody burning around 3.1 kg per kilometre uses roughly 17,200 kg on a 5,550 km transatlantic sector, and that becomes about 54,400 kg — 54 tonnes — of CO₂.

Divided across 180 passengers that is about 302 kg each. The fuel burn calculator does both steps, because the per-passenger figure is the one most comparisons need.

Litres or kilograms, but be consistent

Fuel is bought in litres or gallons and burned in kilograms, and mixing them is the commonest arithmetic error in this subject. Jet A runs about 0.80 kg per litre, so a litre is meaningfully lighter than a kilogram.

The calculator converts before applying the factor. If you are doing it by hand, convert to mass first: the emission factor is per kilogram, and applying it to litres overstates the answer by about 25%.

What the factor does not include

This is combustion CO₂ only — what comes out of the engine. It excludes the emissions from producing and transporting the fuel, which is a lifecycle question, and it excludes aviation's non-CO₂ effects.

Those non-CO₂ effects — contrails and cirrus formation, nitrogen oxides at altitude — are a substantial and genuinely uncertain part of aviation's total climate impact, and are the subject of active research. A CO₂ figure is a floor on the impact, not a complete account of it.

Why SAF does not change this number

Sustainable aviation fuel burns to essentially the same CO₂ per kilogram, because it is chemically similar kerosene. Its benefit is upstream: the carbon in it was recently taken from the atmosphere or diverted from waste, so the lifecycle total is lower even though the tailpipe figure is not.

That distinction confuses a lot of coverage. The SAF calculator models it correctly — same combustion factor, a lifecycle reduction applied to the SAF fraction only.