The average amount of CO2 emitted per km and per person is calculated by the carriers by dividing their energy consumption for the previous year (to which a CO2 emission factor by type of energy is applied) by the number of travelers transported from the previous year and the distance they traveled. This gives an emission factor which will be used in the calculation of emissions on a specific route, depending on the distance of the journey and the mode of transport used.

The method of calculating the distance of a journey is important because it determines the final estimate of CO2 emissions. Find out how this distance is calculated according to the means of transport used:

• In public transportation, the distance traveled is the sum of the distance as the crow flies between each stage of your trip.
• By car, the distance traveled is calculated as the crow flies between the point of departure and arrival.

In public transport, when a journey requires the use of several modes of transport in correspondence, each section is treated individually, then the CO2 emissions are added up.

For example, for a train and then metro journey, the estimated CO2 emission of the journey is the sum of the emissions of the train journey + those of the metro journey, calculated as follows:

Travel distance x CO2 emissions / km of a traveler for this mode of transport.

Our calculator uses the following emissions by mode, which are in 2017 (based on energy consumption and attendance) per traveler:

• by train on the Transilien / RER network: 5.2 g / km
• by tram: 3 g / km
• by metro: 3.4 g / km
• by bus: 96.6 g / km
• by car: 166 g / km

Walking and Vélib 'journeys are not taken into account.