Car Carbon Calculator
Calculate your car's annual CO₂ emissions from annual mileage, fuel type, and fuel efficiency. Use it to compare petrol vs diesel vehicles or assess the benefit of switching to electric.
About this calculator
A car's annual CO₂ output depends on how far it travels, how efficiently it uses fuel, and the carbon content of that fuel. The formula is: Annual CO₂ (kg) = (mileage / efficiency) × fuel_factor, where mileage is in km, efficiency is in L/100km, and fuel_factor is 2.31 kg CO₂/L for petrol or 2.68 kg CO₂/L for diesel. Dividing mileage by efficiency (L/100km) gives total litres consumed over the year (remembering to divide by 100). The fuel factors represent the mass of CO₂ released per litre of fuel combusted — diesel is higher because it is denser in carbon, though diesel engines are often more efficient, so the net comparison depends on both values. Electric vehicles return 0 kg of direct tailpipe CO₂.
How to use
Example: a petrol car driven 15,000 km/year with a fuel efficiency of 7 L/100km. Step 1 — Enter mileage: 15,000 km. Step 2 — Select fuel type: petrol (factor = 2.31 kg CO₂/L). Step 3 — Enter efficiency: 7 L/100km. Calculation: total fuel = (15,000 / 100) × 7 = 1,050 L. Annual CO₂ = 1,050 × 2.31 = 2,425.5 kg CO₂ per year. Switching to a more efficient 5 L/100km petrol model would reduce that to 1,732.5 kg — a saving of nearly 700 kg CO₂ annually.
Frequently asked questions
Why does diesel produce more CO₂ per litre than petrol if diesel cars are more fuel efficient?
Diesel fuel contains more carbon atoms per litre than petrol, which is why combusting one litre of diesel releases 2.68 kg CO₂ versus 2.31 kg for petrol — about 16% more per litre. However, diesel engines extract more energy from each litre of fuel (higher thermal efficiency), so they consume fewer litres over the same distance. In practice, a diesel car typically needs 15–20% less fuel per 100 km than an equivalent petrol model, roughly offsetting the higher per-litre CO₂ factor. The net result is that modern diesel and petrol cars of similar size often have comparable annual CO₂ outputs, with the balance tipping toward diesel on motorway-heavy driving.
How do I find my car's fuel efficiency in L/100km for this calculator?
Fuel efficiency in L/100km is the standard metric in most of Europe, Canada, and Australia. You can find it in your car's owner manual, on the manufacturer's website, or on the window sticker from purchase. If your car displays mpg (miles per gallon), convert using: L/100km = 282.5 / mpg (UK imperial gallons) or 235.2 / mpg (US gallons). Alternatively, you can calculate it empirically: fill your tank, reset the trip counter, drive normally, then at the next fill-up divide litres added by kilometres driven and multiply by 100. Real-world consumption is typically 10–20% higher than official test figures.
What annual CO₂ saving can I expect from switching from a petrol car to an electric vehicle?
An electric vehicle (EV) produces zero direct tailpipe CO₂, so the saving equals your current car's full annual figure — potentially 2,000–3,000 kg CO₂ for an average driver. However, EVs do produce indirect emissions through electricity generation, so the net saving depends on your grid's carbon intensity. On the UK grid (≈0.233 kg CO₂/kWh) and assuming an EV uses about 15 kWh/100km, an EV would emit roughly 348 kg CO₂ per 10,000 km driven — versus around 1,620 kg for a petrol car at 7 L/100km. That is still a saving of over 75%. On a renewable-heavy grid like France or Sweden, EV emissions are close to zero.