Fuel Cost Calculator
Work out what a trip costs in fuel from the distance, your car's fuel economy and the price at the pump — or flip it around and turn a fill-up into your real mpg and L/100 km.
Fuel prices and economy are the numbers you entered — speed, load and weather move real-world results, so yearly figures are estimates.
Formula and calculation
Worked example
A 250-mile trip in a car that does 30 mpg, with gas at $3.50 a gallon, burns 250 ÷ 30 = 8.33 gallons and costs $29.17 — about $0.12 a mile. At 12,000 miles a year the same numbers come to $1,400.00 in fuel.
Why mpg and L/100 km move in opposite directions
Miles per gallon measures distance per unit of fuel; liters per 100 km measures fuel per unit of distance. Because they are reciprocals, a higher mpg is a lower L/100 km, and equal-looking improvements are not equal. Over 100 miles, a car going from 20 to 30 mpg drops from 5 gallons to 3.33 — a saving of 1.67 gallons — while going from 30 to 40 mpg saves only 0.83 gallons. Cost moves with gallons burned, so when you compare two cars, compare the fuel each one burns on the driving you actually do, not the size of the mpg jump.
Getting a trustworthy mpg from a fill-up
The trip-meter method only works if both fills stop at the same point, which is why the pump's automatic shut-off is the usual reference: fill until it clicks, note the gallons, drive, then fill to the click again and read the distance. One tank still includes a lot of noise — a windy day, a roof rack or a lot of city driving — so average three or four fill-ups before you believe the number. Winter blends and air conditioning both pull real-world economy below the label figure, and that gap is normal rather than a fault.
What the results mean
- Trip fuel cost
- What the fuel for this trip costs at the price you entered. Fuel only — tolls, parking and wear are not in this number.
- Fuel needed
- How much fuel the trip burns: US gallons under US units, liters under metric.
- Cost per mile
- Fuel cost for each mile (US units) or kilometer (metric). Handy for comparing a drive against a train fare or an employer's mileage rate.
- Fuel economy used
- Your fuel economy written in both units at once, so a 30 mpg car also shows the 7.84 L/100 km figure.
- Fuel cost per year
- The same fuel economy and price stretched across the yearly distance you entered. Compare it with a monthly transit pass to see which is cheaper.
- Average month
- The yearly fuel cost divided by 12 — an average month for budgeting.
Frequently asked questions
How much gas will a 500 mile trip use at 25 mpg?
500 ÷ 25 = 20 gallons. At $3.50 a gallon that trip costs $70. Enter the same numbers in the trip mode above and you get the fuel, the total cost and the cost per mile in one step.
How do I calculate my mpg from a fill-up?
Divide the miles driven since the last fill by the gallons added. Driving 350 miles on 11.5 gallons is 350 ÷ 11.5 = 30.4 mpg. The second mode does that division, and also reports the same tank as 7.73 L/100 km.
What is 30 mpg in L/100 km?
About 7.84 L/100 km, because 235.214583 ÷ 30 = 7.84. The constant is the exact ratio between the two scales: 100 × 3.785411784 ÷ 1.609344. The same arithmetic works in reverse — 7.5 L/100 km is 31.36 mpg.
Why is my real mpg lower than the window sticker?
The EPA label already discounts the laboratory figure, but it still cannot cover your route. Short trips on a cold engine, stop-and-go traffic, highway speeds above 60 mph, air conditioning and roof racks all push consumption up. Measuring a few tanks the way this calculator does gives a number that matches your own driving.