The formula
cost = distance × consumption ÷ 100 × price per litre + tolls + parking
Where it comes from
The calculation assumes a return trip, which is the usual case. The litres come from multiplying the kilometres by the consumption per hundred and dividing by a hundred; from there, it is all multiplication by price.
How to work it out by hand
- Double the distance if you are coming back
- Multiply the kilometres by the consumption and divide by a hundred
- Multiply the litres by the price of fuel
- Add tolls and parking and divide between the occupants
What is worth knowing
This is the cost of the trip, not the cost of running a car. To compare honestly against the train or a flight you have to add the share of insurance, servicing, tyres and above all depreciation, which is usually the largest line. All in, the real cost sits around 0.25 to 0.35 per kilometre for an average car, well above the 0.10 that fuel alone gives.