You can estimate the carbon dioxide from a car journey with two numbers: how much fuel you burned and how much CO2 each unit of fuel produces. The method is the same for a single trip or a whole year of driving.
How it works
CO2 = fuel used x emission factor. For petrol, burning one litre produces about 2.31 kg of CO2, and one US gallon about 8.89 kg. For diesel, one litre produces about 2.68 kg, and one US gallon about 10.16 kg. These factors are for the fuel burned in the engine only, not for producing and delivering the fuel, which would add more. Fuel used = distance / fuel economy, so keep units consistent throughout.
Step by step
- Write down the trip distance and your car fuel economy in matching units.
- Divide distance by fuel economy to get fuel used.
- Pick the emission factor for your fuel type in matching units.
- Multiply fuel used by the factor to get CO2 in kilograms.
- Divide by 1000 for tonnes if you need that unit.
- For a year, repeat with annual distance instead of trip distance.
Worked example
A petrol car drives 300 km at 7 litres per 100 km. Fuel used = 300 / 100 x 7 = 21 litres. CO2 = 21 x 2.31 = 48.5 kg for the trip. A diesel car on the same trip at 6 litres per 100 km uses 18 litres, and CO2 = 18 x 2.68 = 48.2 kg. Over a year at 15,000 km, the petrol car at 7 litres per 100 km uses 1,050 litres and emits about 2,426 kg, or 2.43 tonnes.
Common mistakes
- Mixing litres with gallons, or miles per gallon with litres per 100 km, in one calculation.
- Using a petrol factor for a diesel car, or the reverse.
- Counting only tailpipe CO2 and presenting it as the full lifecycle footprint.
- Forgetting that real fuel economy differs from the official figure, usually being worse.
Try it with the free tool
You can follow every step above by hand, but the free tool below does the same calculation instantly. Try your own numbers in it, then check one result by hand so you trust the method as well as the answer. Open the full tool here: open the tool.
Checklist before you finish
- Write down every input number first, with its unit, before you calculate anything.
- Work through the steps in order, and do not skip the unit conversions.
- Check the answer against the worked example above: is it the right size, and does the direction make sense?
- Round only at the end, and keep the unrounded figure for any follow-up calculation.
Practice with your own numbers
Pick three cases of your own and work each one by hand. First, try a simple round-number case you can check mentally. Second, try a realistic case with awkward numbers, the kind you will actually meet. Third, try an edge case, such as a very small or very large input, and confirm the answer still behaves sensibly. If any case gives a surprising result, re-check the units and the formula step by step before you trust it.
If your answer looks wrong
Most errors come from a small set of causes. Check that you used the same units throughout, that you divided where the formula divides and multiplied where it multiplies, and that you applied any percentage as a decimal. Check the order of operations: powers and brackets come before multiplication and division, which come before addition and subtraction. Finally, test the method on the worked example in this guide. If your method reproduces the example answer, the method is right and the inputs deserve a second look.
Quick tips
Use your own fuel receipts over a few months for a better economy figure than the brochure number. If you car-share, divide the trip total by the number of people for a per-person figure.