Burning one gallon of gasoline releases about 8.887 kg (19.6 lb) of CO₂; a gallon of diesel about 10.18 kg. The exhaust weighs roughly three times more than the fuel did, because burning bonds each carbon atom to two oxygen atoms drawn from the air. Below are the EPA emission factors for the main fuels, and a calculator for your own mileage or fuel use.
Formula: CO₂ (kg) = quantity × emission factor. In driving mode, gallons = miles ÷ mpg. A mature tree absorbs roughly 21 kg CO₂ a year (EPA), which is what the tree-years figure uses.
| Fuel | Per unit | kg CO₂ | lb CO₂ |
|---|---|---|---|
| Gasoline | gallon | 8.887 | 19.6 lb |
| Diesel | gallon | 10.180 | 22.4 lb |
| Heating oil | gallon | 10.210 | 22.5 lb |
| Propane | gallon | 5.720 | 12.6 lb |
| Jet fuel | gallon | 9.750 | 21.5 lb |
| Natural gas | therm | 5.302 | 11.7 lb |
Tailpipe combustion factors from the US Environmental Protection Agency. Well-to-wheel emissions — adding extraction, refining and distribution — run roughly 15–25% higher.
Petrol is mostly carbon and hydrogen. When it burns, the hydrogen becomes water and every carbon atom bonds with two oxygen atoms pulled from the air to form CO₂. Carbon has an atomic mass of 12; each oxygen atom is 16, so CO₂ weighs 44 — nearly four times the carbon that entered the engine. That is why 2.8 kg of petrol leaves as 8.9 kg of carbon dioxide: the extra mass was already in the atmosphere as oxygen.
Gasoline: A gallon of petrol weighs about 2.8 kg but produces 8.9 kg of CO₂, because each carbon atom picks up two much heavier oxygen atoms when it burns.
Diesel: Denser than petrol and more carbon per gallon — but diesel engines are more efficient, so CO₂ per mile is usually lower.
Heating oil: Essentially the same fuel as diesel (No. 2 distillate), so nearly the same factor.
Propane: Lower carbon per gallon, but a gallon also holds far less energy (91,500 BTU vs 138,500 for oil).
Jet fuel: Kerosene-type Jet A, between petrol and diesel in carbon content.
Natural gas: The lowest-carbon fossil fuel per unit of energy — about 117 lb CO₂ per million BTU against roughly 161 for oil.
About 8.887 kilograms — roughly 19.6 pounds — of carbon dioxide per gallon burned, using the US EPA emission factor. This surprises people because a gallon of petrol only weighs about 2.8 kg. The extra mass comes from the air: burning joins each carbon atom to two oxygen atoms from the atmosphere, and oxygen is heavier than carbon, so the exhaust weighs roughly three times what the fuel did.
Driving 12,000 miles a year in a 27 mpg car burns about 444 gallons, producing roughly 3,950 kg — just under 4 tonnes — of CO₂. At 40 mpg the same mileage produces about 2,670 kg, and at 20 mpg about 5,330 kg. Fuel economy is the single biggest lever for a conventional car.
Per gallon, yes — diesel produces about 10.18 kg against 8.887 for petrol, because it contains more carbon. Per mile it is usually better, since diesel engines are typically 20 to 30 percent more efficient, so they burn fewer gallons to cover the same distance. Which fuel wins depends on the specific vehicles, and diesels carry separate concerns around NOx and particulates.
No. These are tailpipe (combustion) factors — the CO₂ released when the fuel burns. Full well-to-wheel accounting adds extraction, refining, and distribution, which typically raises the total by roughly 15 to 25 percent for petrol and diesel. Electricity comparisons need the same care: an EV's emissions depend entirely on the generation mix of its grid.