The formula
car = litres × fuel factor · train and plane = km × factor per passenger
Where it comes from
A car emits per vehicle and a train or plane per passenger, and that difference changes everything. A car with one person in it is among the worst modes there is; the same car with four is close to the train. A short-haul plane emits more per kilometre than a long-haul one, because take-off — where almost all the fuel goes — weighs far more on a five-hundred-kilometre trip than on a five-thousand-kilometre one.
How to work it out by hand
- Work out the litres the car uses on that journey
- Multiply them by the fuel factor: 2.31 kg per litre of petrol
- Divide by the number of occupants for the per-person footprint
- Compare with train and plane, which are already counted per passenger
What is worth knowing
The factors are European averages and come from the same place as the personal footprint calculator, so the site never carries two different numbers for the same thing: 2.31 kg of CO₂ per litre of petrol, 2.68 per litre of diesel, 0.035 kg per passenger kilometre by train and between 0.15 and 0.156 by plane. The electricity factor depends entirely on the national grid, so it is left as a field: in France or Norway an electric car emits a fraction of what it emits in Poland.