How it works
The math behind a fuel estimate has only three moving parts: distance, efficiency and price. The calculator first works out the distance you will actually drive. If you tick round trip, the one-way miles are doubled. Next it divides that distance by your miles per gallon to get the gallons you will burn, and finally it multiplies gallons by the price per gallon.
Gallons = miles driven ÷ MPG Fuel cost = gallons × price per gallon
Two extra figures are shown. Cost per mile is the price per gallon divided by MPG, which is handy when you are comparing a drive with a train ticket or a mileage reimbursement rate. Cost per person splits the fuel bill evenly across everyone riding along. Nothing is rounded until the final display, so the per-person amounts add back up to the total within a cent.
Methodology and assumptions
- Fuel economy is treated as constant for the whole drive. Real MPG moves with speed, terrain, cargo, tire pressure, air conditioning use and weather.
- The price you enter is applied to every gallon. On a long route, prices change between states, so a blended average of the places you expect to fill up is the most realistic input.
- Only fuel is counted. Tolls, parking, wear and tear, depreciation and food are not included. For the full cost of driving a mile, see the car ownership cost calculator.
- Electric vehicles can be estimated by entering miles per kWh as “MPG” and the price per kWh as the “gas price”; the formula is identical.
Worked examples
Weekend getaway: 450 miles each way, 28 MPG, $3.25 a gallon, round trip, three friends
Round trip doubles the distance to 900 miles. Dividing by 28 MPG gives 32.14 gallons, and at $3.25 a gallon the fuel bill is $104.46. Split three ways, each person owes $34.82. Every mile costs about $0.12 in fuel.
Moving across the country: 1,200 miles one way in a 22 MPG SUV at $3.60
The SUV burns 54.55 gallons, for a fuel cost of $196.36 ($0.16 a mile). Making the same drive in a 35 MPG hybrid would use 34.29 gallons and cost $123.43, a saving of $72.94.
How to get a realistic MPG number for a trip
The window-sticker rating is a lab figure. The EPA publishes separate city, highway and combined ratings, and most road trips are mostly highway, so the highway number is usually the better starting point. Even that can be optimistic at 75 mph or with a roof box, a full trunk and four passengers. If you track your own fill-ups, use your real highway average instead. Our MPG calculator turns the odometer and pump readings from one tank into an exact figure.
A quick way to build in a cushion is to knock 10% off the rated highway MPG for heavy loads, mountain passes or sustained high speeds. On the 900-mile example above, dropping from 28 to 25.2 MPG raises the bill from $104.46 to $116.07.
Ways to spend less on fuel on the road
- Ease off the highway speed. Aerodynamic drag rises sharply with speed, so cruising at 65 instead of 75 can noticeably improve economy on most cars.
- Check tire pressure before you leave. Under-inflated tires add rolling resistance and wear. Inflate to the pressure on the driver’s door sticker, not the maximum on the sidewall.
- Pack light and ditch the roof rack. Weight and anything that sticks up into the airflow both cost fuel.
- Plan where you fill up. Prices can differ noticeably between states and between stations a short distance apart, so a fuel-price app can pay off on a long route.
- Use cruise control on flat roads. Steady throttle beats constant speeding up and slowing down.
- Compare vehicles. If you have two cars, run the trip in both. The difference is often larger than people expect, as the moving example shows.
Splitting gas money fairly
An even split is the simplest arrangement and is what the per-person line shows. Some groups prefer to have riders cover fuel while the driver covers the car itself, since the owner is already paying for insurance, depreciation and maintenance. If you want to reimburse a driver for the whole cost of using the car, a per-mile rate that includes those costs is fairer than fuel alone. That rate is usually several times the fuel cost per mile.
Frequently asked questions
How do I calculate gas cost for a trip?
Divide the miles you will drive by your car’s miles per gallon to get gallons, then multiply by the price per gallon. For a round trip, double the one-way distance first. The calculator does all three steps and also shows the cost per mile and per person.
Should I use city, highway or combined MPG?
For long drives on interstates, start with the highway rating or, better, your own highway average. For trips that are mostly in town, use the city rating. Combined MPG is a reasonable middle ground when you are not sure.
Does a round trip just double the cost?
Yes. With the same route, vehicle and gas price both ways, the fuel cost is exactly twice the one-way cost. If you expect different prices on the way back, run the calculator once for each leg.
How much does it cost to drive 100 miles?
At 28 MPG and $3.25 a gallon, 100 miles uses 3.57 gallons and costs $11.61. Change either input and the cost moves in direct proportion to price and inversely with MPG.
Can I use this for an electric car?
Yes. Enter your efficiency in miles per kWh in the MPG box and your electricity price per kWh in the price box. The calculator’s formula is the same: distance divided by efficiency, times price.
Does the estimate include tolls or wear on the car?
No. It counts fuel only. Tolls, parking, oil changes, tires and depreciation are separate costs. Our car ownership cost calculator rolls those into a full cost per mile.
Why is my real fuel bill higher than the estimate?
Common reasons are higher speeds than you planned for, idling in traffic, a heavier load, headwinds, running the air conditioning hard or paying more than your assumed price at some stops.