Vehicle carbon dioxide estimate:
formula and worked example

Why fuel use maps so directly to CO2

When a petrol or diesel engine burns fuel, the carbon in it ends up as carbon dioxide. Because the carbon content of a litre of a given fuel is roughly fixed, the CO2 released is almost exactly proportional to the volume burned. That is unlike other pollutants, which depend on engine technology and filters.

This makes the calculation unusually simple and reliable. If you know how much fuel a vehicle used, you can estimate its CO2 without knowing anything about the engine, the route or the driver, provided you pick the right factor for the fuel.

One consequence is that driving style shows up in the answer only through fuel burned. A driver who accelerates hard and one who coasts will have different footprints because they burn different litres, not because the carbon content of a litre differs.

The formula and its two inputs

• Fuel litres: the volume actually consumed, from fuel receipts or distance divided by km per litre.
• Emission factor: kilograms of CO2 released per litre of that specific fuel. Petrol is commonly quoted at a little over 2.3, and diesel is higher per litre, but check the source your reporting framework requires.
• Result: kilograms of CO2 from combustion.

If you only have distance and economy, you can find litres first by dividing distance by km per litre. A 600 km trip at 15 km per litre uses 40 litres, which is the volume in the worked example. The factor then does the rest.

Keep the unit of the factor in view. Some sources quote kg CO2 per litre and others quote kg per gallon or grams per kilometre. Dividing and multiplying across those without converting is the quickest way to end up out by a large margin.

Worked example: 40 litres of petrol

A driver fills 40 litres of petrol, and the emission factor used is 2.31 kg CO2 per litre.

1. 1Fuel burned: 40 litres.
2. 2Factor: 2.31 kg per litre.
3. 3Multiply: 40 × 2.31 = 92.4.
4. 4The tank emits about 92.4 kg of CO2 when it is burned.

If those 40 litres covered 600 km, the footprint is 92.4 ÷ 600 = 0.154 kg per km, or 154 grams per kilometre, which is the unit most vehicle comparisons use.

For a sense of scale, 92.4 kg is roughly what a 40-litre tank costs the atmosphere each time you fill up, so a driver filling once a week produces a little over 4.8 tonnes a year from that car alone. Cutting even one in ten of those litres takes about half a tonne off the total.

Putting the number in context

A figure in kilograms is easier to grasp when converted to something else: 92.4 kg is about a tenth of a tonne, and a family car driven a typical annual distance will emit a few tonnes. Comparisons between vehicles are best made per kilometre, since total litres depend on how far each is driven.

• Per-km values let you compare a hatchback with an SUV on equal footing.
• Per-passenger values divide the trip total by occupants and show the benefit of sharing.
• Annual totals help set a baseline for a reduction target.

Remember also that the calculation sees only carbon dioxide. A fuller greenhouse-gas picture would include small amounts of methane and nitrous oxide from the exhaust and the emissions from making and moving the fuel. Those are real, but the CO2 from combustion is by far the largest part, which is why this simple estimate is widely used.

How the fuel volume moves the result

Ten percent more fuel gives exactly ten percent more CO2: 92.4 becomes 101.64. The only ways to lower the number are to burn less fuel or to switch to a fuel with a lower factor. Driving smoothly, keeping tyres inflated and removing unneeded weight all cut litres, and the carbon follows automatically.

Common slips in vehicle CO2 estimates

• Using the petrol factor for diesel, or the reverse, which changes the answer by more than a tenth.
• Mixing gallons and litres when reading receipts from different countries.
• Treating the result as a full lifecycle number, when extraction, refining and transport are not included.
• Applying a litres-based factor to electric or hybrid energy, which needs a different calculation.
• Quoting kilograms of CO2 and tonnes of CO2e interchangeably, which is not the same measure.

Using the estimate in practice

A simple way to apply it is to record fuel litres for a month, multiply by the factor, and keep a running total. Over a year you get a personal baseline against which a change of driving habit, a different car, or a switch to public transport can be measured.

• Gather fuel receipts or litres from the trip log.
• Choose the factor for the right fuel and write down its source.
• Multiply litres by the factor.
• Divide by distance for grams per kilometre.
• Repeat monthly to see the trend.