Transport Carbon Footprint: How Mode Comparisons Are Calculated

For every mode the method is the same:

1. Distance — how far that mode actually travels for your trip. A return trip counts each one-way distance twice.
2. Mode — the type of transport, which selects the factor.
3. Factor — kilograms of CO₂e per passenger-km (trains, coaches, flights) or per vehicle-km (cars).
4. Scope — what the factor includes and excludes.
5. Result — distance × factor, divided by the people in the car when the factor is per vehicle.

The factor is not fixed for a mode: it depends on fuel, vehicle efficiency, occupancy, the electricity source for electric vehicles and trains, flight distance and class, and the lifecycle boundary. To run it for your own trip, use the Flight vs Train CO₂ calculator.

The calculator uses one source for every mode: the UK Government GHG Conversion Factors for Company Reporting 2025 (DESNZ), flat-format file. All values are kg CO₂e (carbon dioxide equivalent, which also counts methane and nitrous oxide). This table is generated from the same data file as the calculator, so the two cannot disagree:

The factors describe emissions from operating the vehicle, not a full lifecycle:

• Flights, rail, coach, petrol and diesel cars — emissions from the fuel burned or the electricity used to run the service. The separately published well-to-tank (fuel supply) factors are not added.
• Battery electric car — UK electricity generation plus transmission and distribution losses, which is how the source builds its electric-car factor.
• Not included — building vehicles, batteries, track, roads and airports.

The ground factors are UK averages. A train running on a different country's grid, or a coach with different occupancy, can have a very different factor, so treat the results as UK-average estimates unless you have a local factor.

Car factors are per vehicle-km, so the car's total is divided by the number of people in it. Sharing a car does not change the car's emissions, only each person's share. The UK average factors stand in for an unknown car; for your own petrol or diesel car, litres burned × a fuel factor is more accurate, and the Carbon Footprint calculator works from your own fuel use.

For an EV, operational emissions are electricity drawn × an electricity emission factor:

EV emissions (kg CO₂e) = kWh drawn from the grid (including charging losses) × grid factor (kg CO₂e/kWh)

The UK average battery-electric car factor above (0.04047 kg CO₂e per vehicle-km) already embeds UK grid electricity for its year. On a different grid, or with your own consumption, use your kWh and a local factor; choosing that factor is covered in the electricity CO₂ guide.

Manufacturing emissions — including the battery — are a separate lifecycle question. Keep them out of an operational comparison, or add them for every vehicle with a clearly stated source and boundary; do not add them for one car only. Running cost is a different question again, covered in the EV vs petrol running cost guide.

Rail, coach and bus factors are already per passenger-km: the source has spread the vehicle's emissions over its average passengers, so the occupancy assumption is built into the number and it is not divided again. A busy service performs better than the average and an empty one worse, but you cannot usually know the load in advance, so the published average is the practical choice. The UK source also publishes separate local-bus factors; the calculator compares rail and coach only.

The UK source publishes flight factors in four bands — domestic (within the UK), short-haul (UK to/from Europe), long-haul (UK to/from outside Europe) and international (between non-UK countries) — each with and without non-CO₂ effects. The calculator uses the average-passenger factor because seat class is often unknown; the source gives higher factors for business and first class because those seats take up more of the aircraft.

Between the same two cities, the flight, rail line and road are different lengths. Rail and road routes are usually longer than the straight-line flight distance. Comparing every mode over one shared distance can therefore flatter or penalise a mode. The calculator asks for two one-way distances — the flight distance and the rail or road distance — and doubles both for a return trip. Use a route planner or timetable for the rail or road distance.

Illustrative trip, not a specific route: return journey, flight distance 340 km one way, rail or road distance 460 km one way, short-haul flight with non-CO₂ effects, two people in the car. Each line is trip distance × factor, rounded to 0.1 kg:

The result is an estimate for an average UK vehicle or service. It does not cover journeys to and from the station or airport, actual load on your train or flight, a specific car's fuel economy, or emissions from manufacturing. Buying carbon offsets is a separate action: it does not change the emissions of the trip itself.

Related questions with their own guides:

• Your whole footprint — the carbon footprint guide.
• Reducing travel emissions as part of a plan — the carbon reduction planning guide.

The Carbon Footprint hub collects the carbon tools; the methodology page documents the sources.