Sharing a car to work or on a long trip cuts the cost for everyone, takes cars off the road and, in many cities, gets you into the carpool lane. The fair way to share is to split what the trip really costs the driver: fuel, tolls and parking, plus a share of the wear on the car.
How the cost is worked out
Fuel per round trip = 2 × one-way distance ÷ fuel economy (mpg)
Cost per round trip = fuel × price per gallon + 2 × distance × wear per mile + tolls and parking
Share per person = cost per round trip ÷ number of people in the car
The calculator counts the driver as one of the people sharing, so everyone pays the same share. If the driver gives the others a ride as a favor, simply split between passengers by lowering the number of people.
What wear per mile should I use?
Fuel is only part of what a mile costs. Tires, oil changes, repairs and depreciation add roughly 10 to 25 cents a mile for a typical car. The IRS business mileage rate (70 cents a mile in 2025) covers everything including insurance and depreciation, so it is a ceiling rather than a fair split for a commute. Many carpools use fuel plus 5 to 10 cents a mile.
Example
Three coworkers share a 20-mile commute each way in a 30 mpg car with gas at $3.20 a gallon, $5 of tolls and parking a day and 10 cents a mile of wear. A round trip uses 1.33 gallons and costs $13.27, or $4.42 each. Over 20 working days, each pays about $88 a month instead of $265 driving alone, saving about $177. The group also avoids nearly half a ton of CO2 a month.
Tips for a smooth carpool
- Agree up front on the route, the waiting time and how often money changes hands; payment apps make weekly settling easy.
- Rotate drivers if several people have cars, so the wear is shared and nobody has to keep track of money.
- Check your insurance: sharing costs is fine with most personal policies, but charging more than the cost can count as a commercial ride.
- Look for perks: many employers and transit agencies offer preferred parking, guaranteed rides home and ride-matching apps for carpoolers.
Limits
The CO2 figure assumes each rider would otherwise drive a car with the same fuel economy, using 8.89 kg of CO2 per gallon of gasoline. Real savings depend on the detour to pick up riders, which you can add to the distance.