How to calculate travel carbon emissions

Start with distance and an emission factor

A travel estimate brings together two things: how far you travel and an emission factor for that type of journey. The factor expresses an average quantity of emissions for each kilometre travelled.

The distinction matters. A rail or flight factor already represents one passenger’s share. A car factor represents the vehicle, so the journey total is divided by the occupants when showing each person’s share.

Our travel emissions calculator applies the relevant 2026 UK Government factors and shows travel operations and fuel or energy supply separately. Flights can also include a separate estimate of additional warming.

Read the unit as well as the number

Carbon dioxide equivalent, or CO₂e, is a common unit used to express the effect of different greenhouse gases. A result in kilograms of CO₂e is therefore different from a result that counts carbon dioxide alone.

There are 1,000 kilograms in one tonne. A journey estimated at 250 kg CO₂e is 0.25 tonnes CO₂e. Converting the unit does not change the emissions or make an estimate more precise.

Our calculator presents larger totals in tonnes and retains the underlying calculation when carrying an estimate into an enquiry. The optional aviation warming allowance is identified separately because it extends beyond the standard greenhouse gas total.

See how the calculation works

These examples use our calculator’s 2026 factors. Each is a 100 km one-way journey. The rail example is for one passenger; the car examples are for one average-size vehicle.

Car figures use UK averages, including the UK electricity mix for the electric car. These are illustrative estimates, not measurements of a particular vehicle or train.

A flight example

For one passenger flying 1,000 km one way in economy on a UK short-haul route, our calculator gives 96.84 kg CO₂e for operations and fuel supply. Selecting additional aviation warming adds 51.41 kg, bringing the displayed estimate to 148.25 kg. The flight calculation guide explains this separate allowance.

Account for the whole journey once

One way or return

Enter the one-way distance, then select return if you are making the same journey back. The calculator doubles that distance. Entering a return distance and selecting return would count the journey twice.

People travelling

For rail and flights, the group total increases with the number of passengers. For one car, the vehicle total stays the same when you change the number of occupants; each person’s share changes. Two people sharing the petrol car above have a total of 20.751 kg CO₂e, or approximately 10.38 kg each.

Miles and kilometres

Select the unit that matches your distance. We convert one mile to 1.609344 kilometres before applying the factors. A 100-mile return journey is therefore 321.8688 km, not 200 km.

Route distance

Use the actual road or rail route distance for land travel. For flights, use direct airport-to-airport distance and calculate connecting legs separately. The published flight factors already contain a distance allowance.

Understand what the estimate covers

The calculator includes travel operations and the supply of fuel or electricity. Electric and plug-in hybrid car estimates also include electricity transmission and distribution. A battery electric car can therefore have an emissions estimate even though it has no exhaust emissions while driving.

Vehicle manufacture, stations, airports, roads, accommodation and other activities are outside this calculator. It is a journey estimate, rather than a full assessment of a trip or a vehicle’s life cycle.

Car and domestic rail factors use UK averages. The international rail option uses a Eurostar reference, which should not be treated as an average for every overseas rail service. Real journeys vary with vehicle, occupancy, route, energy source and operating conditions.

Keep the factor year and the assumptions alongside your result. If you are comparing journeys, use the same boundaries and traveller numbers, and include each journey leg that matters to the comparison.

Use the estimate to inform your next step

A useful estimate helps you identify choices. Consider whether a journey is needed, whether rail is practical, whether people can share a car, or whether meetings can be combined. Compare realistic alternatives for the whole journey.

If you are recording business travel, retain the underlying journey data and method. A travel estimate alone is not a complete organisational footprint. Our carbon management team can help define the wider scope and the information needed.

If you want to support verified carbon projects, you can carry your calculator result into an individual or business purchase-options enquiry. The team will review quantities, project availability and retirement arrangements with you. A calculated footprint does not automatically determine a purchase quantity or establish that a journey is carbon neutral.

Read how to buy carbon credits for the questions to ask before purchasing.

Sources and methodology

The examples and calculator use the Department for Energy Security and Net Zero’s 2026 greenhouse gas conversion factors, including the revised July flat file, version 1.2.

• Full factor workbook (Excel) - factors, category definitions and worksheet guidance.
• 2026 methodology report (PDF) - the basis and limitations of the published factors.

Factors and worked examples checked against the calculator on 2 October 2026. Contains public sector information licensed under the Open Government Licence v3.0.