Car Emissions Calculator
Calculate the CO₂ emissions of any petrol or diesel car journey based on fuel efficiency, distance, and the number of passengers sharing the ride. Perfect for comparing routes or travel modes.
About this calculator
When a vehicle burns fuel, each litre releases a fixed quantity of CO₂ depending on the fuel's carbon content. Petrol (gasoline) releases 2.31 kg CO₂ per litre burned, while diesel releases 2.68 kg per litre — diesel's higher carbon density is why it emits more per unit volume despite better fuel economy. The number of litres burned equals distance divided by fuel efficiency (in MPG, converted appropriately). Dividing total vehicle emissions by passengers gives each person's share. The formula is: CO₂ per person = (distance / fuelEfficiency) × fuelEmissionFactor / max(passengers, 1), where fuelEmissionFactor = 2.31 for petrol or 2.68 for diesel (kg CO₂/gallon). Carpooling is one of the simplest ways to halve or quarter per-person emissions on any given journey.
How to use
Example: a 150-mile petrol car journey at 35 MPG with 2 passengers. Step 1 — Gallons burned: 150 / 35 = 4.286 gallons. Step 2 — Total CO₂: 4.286 × 2.31 = 9.90 kg. Step 3 — Per person: 9.90 / 2 = 4.95 kg CO₂ each. Compare with a solo diesel driver doing the same trip at 45 MPG: 150 / 45 × 2.68 / 1 = 8.93 kg — more per person despite the more efficient engine, because no passengers share the load.
Frequently asked questions
Why do diesel cars emit more CO₂ per litre than petrol cars despite better fuel economy?
Diesel fuel has a higher energy density and a higher carbon content than petrol, so combusting one litre of diesel releases more CO₂ (2.68 kg) than one litre of petrol (2.31 kg). However, diesel engines are thermodynamically more efficient, meaning they travel further on each litre. The net result depends on the specific vehicles compared: a modern diesel often produces slightly less CO₂ per mile than a comparable petrol model, but the advantage has narrowed as petrol engine technology has improved. Diesel's real-world advantage also depends heavily on driving style and whether the vehicle is used predominantly for motorway or city driving.
How much does carpooling reduce per-person car emissions?
Carpooling divides total vehicle emissions equally among passengers, so adding one passenger to a solo driver instantly cuts per-person emissions by 50%. Two additional passengers reduce each person's share to one-third of the solo figure. This makes carpooling one of the most cost-effective and immediate emission reduction strategies available for road travel, with no change to the vehicle or fuel required. Apps and employer commute schemes can systematically enable carpooling for regular journeys, compounding savings over time.
How do car emissions compare to other modes of transport for the same journey?
Per passenger-kilometre, cars sit in the middle of the transport emission spectrum. Long-haul flights and solo car journeys have the highest per-person footprints, while intercity trains, coaches, and car-sharing fall significantly lower. A UK average car emits roughly 0.17 kg CO₂ per passenger-kilometre for a solo driver, compared to about 0.04 kg for a national rail journey and 0.10 kg for a domestic flight. Filling the car with passengers, or switching to an electric vehicle charged on low-carbon electricity, can bring car travel close to or below rail emissions.