Car Carbon Calculator
Estimate annual CO₂ emissions from a passenger car given your annual mileage, fuel type, and fuel efficiency. Returns kilograms of CO₂ per year.
Last updated: May 2026
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About this calculator
The formula computes Annual CO₂ (kg) = (mileage / efficiency) × fuelEmissionFactor, where mileage is annual kilometres driven, efficiency is the fuel-economy input, and fuelEmissionFactor is kg CO₂ per litre of fuel burned (petrol 2.31, diesel 2.68, EV proxy 0.5, walking/cycling 0). Note: although the UI labels efficiency as 'L/100km', the formula uses it directly as a divisor of mileage, which mathematically corresponds to km/L (the inverse). A car using 8 L/100km has a real efficiency of 12.5 km/L; entering 8 produces a litre figure of 1,500 L for 12,000 km, while the true value is 960 L — so the calculator over-states fuel burn (and emissions) by about 56% if you enter the L/100km value literally. To match real emissions, enter the inverse: for 8 L/100km, input 12.5. Edge cases: the formula assumes constant fuel economy, but real-world efficiency depends heavily on driving style, traffic, weather, and trip length — city driving can be 20–40% less efficient than highway. Petrol factor 2.31 kg/L includes combustion only; lifecycle (well-to-wheel) emissions add roughly 20% for extraction and refining. EV emissions vary 20× across grids (Iceland near zero, India ~150 g/km); the calculator's 0.5 placeholder is a rough world-average and should be replaced by grid-specific data for accuracy.
How to use
Example 1 — petrol car, treating efficiency as km/L. Annual mileage 12,000 km, efficiency 12.5 km/L (real-world for 8 L/100km), fuel petrol (2.31). Step 1: litres burned = 12,000 / 12.5 = 960 L. Step 2: emissions = 960 × 2.31 = 2,218 kg CO₂. Verify: a typical mid-size petrol car driven 12,000 km/year emits about 2.0–2.5 tonnes CO₂, matching this result ✓. If you instead enter 8 (the L/100km value), the formula computes 12,000/8 × 2.31 = 3,465 kg — about 56% too high, because the formula treats the input as km/L not L/100km. Example 2 — diesel car. Annual mileage 20,000 km, efficiency 16.7 km/L (real-world for 6 L/100km), fuel diesel (2.68). Step 1: litres burned = 20,000 / 16.7 ≈ 1,198 L. Step 2: emissions = 1,198 × 2.68 ≈ 3,210 kg CO₂. Verify: a 20,000 km/year diesel car at 6 L/100km should emit roughly 3.2 tonnes; published average for European diesel hatchbacks driven moderately heavily falls in this range ✓. Note diesel's higher per-litre CO₂ (2.68 vs 2.31) is partially offset by its higher fuel economy; for total per-km emissions, diesel and petrol are often within 10% of each other for similar-class cars.
Frequently asked questions
Why does the calculator treat the efficiency input as km/L when the label says L/100km?
The formula divides mileage by the efficiency input, which is mathematically the operation you do with km/L (kilometres ÷ kilometres-per-litre = litres) rather than with L/100km. To get litres from L/100km, you would multiply mileage by efficiency and divide by 100. The label suggests one but the code does the other — a UI bug that this calculator inherits. To get a correct result, convert your L/100km to km/L before entering: km/L = 100 / (L/100km). So 8 L/100km → 12.5 km/L; 6 L/100km → 16.7 km/L; 10 L/100km → 10 km/L (numbers happen to match for 10). Alternatively, US users who already think in MPG can convert MPG directly to km/L by multiplying by 0.4251 (so 30 MPG ≈ 12.75 km/L, matching ~7.8 L/100km). The calculator is unable to detect the unit mismatch, so it's on the user to enter the correctly-converted figure.
How do petrol and diesel compare on emissions per km?
Diesel has higher per-litre CO₂ emissions (2.68 vs 2.31 kg/L for petrol, ~16% higher) because diesel fuel has more carbon atoms per litre than petrol. However, diesel engines are typically 20–30% more fuel-efficient than equivalent petrol engines, meaning they burn fewer litres per km. The net effect is that for similar-class cars, diesel and petrol per-km CO₂ emissions are usually within 5–15% of each other, with diesel slightly lower in many cases. Diesels also emit more NOx and particulates (mitigated by modern after-treatment systems), which have local air-quality impacts not captured in CO₂ accounting. Hybrid petrol cars (Toyota Prius, etc.) are typically 30–50% lower per km than equivalent non-hybrids. Plug-in hybrids are highly variable — if you charge from the grid and drive mostly on electricity, lifecycle emissions can be 50–70% lower than a non-hybrid; if you rarely charge them, they perform like a slightly heavier conventional car.
Are EVs really zero-emission?
Not in well-to-wheel terms, but typically much lower than petrol or diesel. EVs themselves emit no tailpipe CO₂, but the electricity used to charge them does have an emissions footprint that varies enormously by grid mix. In Iceland (geothermal/hydro): near zero per km. In Norway (mostly hydro): under 10 g/km. In the UK (mixed, 2024 mix): roughly 50 g/km. In the US average: roughly 80–120 g/km. In coal-heavy grids like Poland, India, China: 150–200 g/km — still lower than petrol's ~180 g/km tailpipe but with smaller margins. Additionally, EVs have higher manufacturing emissions (the battery especially adds 5–10 tonnes CO₂), but over a typical 200,000-km lifetime an EV pays back this 'carbon debt' within 1–3 years on most grids. The 0.5 placeholder factor in this calculator is a rough mid-range number; for accurate EV accounting, look up your specific utility's published grid carbon intensity and the car's manufacturer-rated kWh/100km consumption.
What are the common mistakes when estimating car emissions?
The biggest mistake here is the unit confusion described above — entering L/100km values literally when the formula expects km/L. Convert before entering. The second is using the manufacturer-rated WLTP or EPA fuel economy, which is lab-tested and typically 15–25% better than real-world consumption; for accurate emissions, use your actual fuel-economy logbook or fuel-trip-computer figures over the last 12 months. The third is ignoring driving style: aggressive acceleration, high-speed cruising (>110 km/h), short trips with cold engine, and rooftop cargo can each add 10–25% to fuel consumption. People also forget that EV emissions depend on when you charge (grid is cleaner mid-day with solar, dirtier evening with peakers) and where (state/region matters). For multi-driver households, also remember that emissions are per car, not per person — a couple sharing one car emits no more than a single driver of that car, while two single-occupant cars double the emissions.
When should I not use this calculator?
Do not use it for hybrid or plug-in hybrid vehicles without understanding their dual-fuel nature; for plug-in hybrids, you would need to separately estimate the petrol-only mileage and electric-only mileage and run the calculator on each component. It is not appropriate for motorcycles, RVs, or vans with significantly different efficiency curves and emission factors. Do not use it for commercial fleet operations or freight trucking, which need GHG Protocol Scope 1 reporting with VECTO or similar regulatory tools. It does not handle biofuel blends (E10, E85, B20) correctly; pure biofuel emission factors differ from fossil equivalents (E85 is roughly 70% lower in well-to-wheel terms but the calculator has no biofuel option). For corporate sustainability or carbon-credit reporting, use official factors from EPA, DEFRA, or your jurisdiction's GHG inventory guidelines, not generic averages. Finally, the EV proxy of 0.5 is a rough world-average; for actual EV decisions look up your specific car's manufacturer-rated kWh/100km and your grid's carbon intensity.