The estimated costs share in an offer or request is a nonbinding recommendation for determining travelling costs resulting from fuel consumption. This is usually meant for a single trip (one direction) with one passenger - unless specified otherwise. More passengers would lower the costs.
Starting from fuel consumption and the number of people in the car, you can calculate your own estimation in the form below.
Fuel costs alone do not cover the costs of driving and maintaining a car by far. Yet, this highlights the advantages of carpooling. Assuming a cost of 20 cents per kilometre, on a distance of 50 km (100 km for the round trip), driving and maintaining a car would cost 400 EUR a month (assuming 20 working days). Driver and passengers can make an agreement that they do not just share fuel costs, but also maintenance affordance like taxes, insurance, or repair costs. Without a carpool, the driver has to pay for all this by him- or herself. It would be best, if all carpool members agree on a flat price (like 20 cents per kilometre), which covers all costs.
One person's share would then be determined via the following, straight formula: (flat price * travelled kilometres) / number of people in the car (in the simplest case 2 people)
Driving: 9,0 cents per km
Repairs: 2,8 cents per km
Value lost: 15,6 cents per km
Fixed costs: 4,9 cents per km
Total: 32,3 cents per km
10% have been added for mainatenance costs.