Last updated: 2026-07-02

What a driving carbon footprint measures

The carbon footprint of driving is the amount of carbon dioxide released when a car burns fuel, expressed in kilograms of CO₂. It depends on three things: how far you drive, how much fuel the vehicle uses per kilometre, and which fuel it burns. This calculator reports the tailpipe (tank-to-wheel) figure — the CO₂ that comes out of the exhaust.

How the estimate is built

The calculation has two steps.

Step 1 — fuel used. Fuel economy is expressed in litres per 100 km, so the fuel burned over a distance is:

Fuel (L) = distance (km) ÷ 100 × fuel economy (L/100km)

Step 2 — CO₂ emitted. Every litre of fuel burned releases a fixed amount of CO₂, set by the fuel's carbon content:

CO₂ (kg) = fuel (L) × combustion factor (kg CO₂/L)

The combustion factors are the standard values used by the UK DEFRA Government GHG conversion factors and the US EPA: about 2.31 kg CO₂ per litre of petrol and 2.68 kg CO₂ per litre of diesel. Diesel emits more per litre because it is denser and carries more carbon.

Worked example

Consider a year of driving: 15,000 km in a petrol car that uses 7 L/100km.

• Fuel used: 15,000 ÷ 100 × 7 = 1,050 L.
• CO₂: 1,050 L × 2.31 kg/L = 2,425.5 kg CO₂ — roughly 2.4 tonnes for the year.

If the same car were diesel using 6 L/100km instead:

• Fuel used: 15,000 ÷ 100 × 6 = 900 L.
• CO₂: 900 × 2.68 = 2,412 kg CO₂.

The two come out close: diesel emits more per litre but the diesel car burns fewer litres, so the totals are similar.

Typical annual footprints for context

These are illustrative; the real figure shifts with the vehicle, how it is driven, and conditions.

Tailpipe versus lifecycle

This model counts only combustion at the exhaust. A fuller lifecycle (well-to-wheel) accounting would add:

• Well-to-tank emissions from extracting, refining, and transporting the fuel — typically another 15–25%.
• Vehicle manufacturing and disposal, spread across the car's lifetime.

For electric vehicles the tailpipe figure is zero; their footprint depends instead on how the charging electricity is generated.

Ways to lower the footprint of driving

• Drive less and combine trips. Distance is a direct multiplier on emissions.
• Choose a more efficient vehicle. Fewer litres per 100 km means proportionally less CO₂.
• Drive smoothly. Moderate speeds, steady acceleration, and correctly inflated tyres cut fuel use.
• Consider electric or shared transport for routine journeys where it fits.

Limitations

• Factors are standard averages. Actual emissions vary with driving style, load, terrain, weather, and vehicle condition.
• Tailpipe only. Upstream fuel production and vehicle manufacturing are not included.
• Constant economy assumed. Real fuel economy differs between city and highway driving; use a representative figure.

Use this calculator as a rough awareness tool for comparing journeys, vehicles, and fuels, not as a certified emissions measurement.