How to use the gas mileage calculator
- Choose whether you know the odometer readings or just the distance driven.
- Choose miles or kilometers.
- Enter the odometer reading at your last fill-up and the reading now, or the distance you drove.
- Enter the fuel added to fill the tank again, in US gallons, UK gallons or liters.
- Optionally enter the fuel price to see the cost per mile or kilometer.
- Read your fuel economy in all four common units.
How fuel economy is calculated
The fill-up method measures what your car actually uses. Fill the tank, note the odometer, drive normally, then fill it again. The fuel it takes the second time is exactly what you burned over that distance.
The calculator converts with exact factors: 1 mile is 1.609344 km, a US gallon is 3.785411784 liters and a UK gallon is 4.54609 liters. Because the UK gallon is larger, the same car scores about 20% higher in UK mpg than in US mpg.
Worked example
The defaults are odometer readings of 12,450 and 12,800 miles and 12.5 US gallons to refill, at $3.50 a gallon:
- Distance: 12,800 − 12,450 = 350 miles
- Fuel economy: 350 ÷ 12.5 = 28 mpg (US)
- In liters: 12.5 × 3.785411784 = 47.32 L over 563.27 km, so 47.32 ÷ 563.27 × 100 = 8.40 L/100 km
- In UK gallons: 47.32 ÷ 4.54609 = 10.41, so 350 ÷ 10.41 = 33.6 mpg (UK)
- Cost of the tank: 12.5 × $3.50 = $43.75, or $43.75 ÷ 350 = $0.125 a mile
A metric example: 500 km on 40 liters is 40 ÷ 500 × 100 = 8 L/100 km, or 12.5 km/L.
Fuel economy conversion table
| mpg (US) | mpg (UK) | L/100 km | km/L |
|---|---|---|---|
| 15 | 18.0 | 15.68 | 6.38 |
| 20 | 24.0 | 11.76 | 8.50 |
| 25 | 30.0 | 9.41 | 10.63 |
| 30 | 36.0 | 7.84 | 12.75 |
| 40 | 48.0 | 5.88 | 17.01 |
| 50 | 60.0 | 4.70 | 21.26 |
To convert yourself: L/100 km = 235.215 ÷ mpg (US), and mpg (UK) = mpg (US) × 1.20095.
Getting an accurate figure
Fill to the same point every time. Stopping at the first automatic click of the pump is the easiest way to be consistent. Don't top off after the click, since the extra fuel can spill or end up in the vapor recovery system. One tank can be skewed by a long highway run or a week of short trips, so average three or more fill-ups. Add up the miles and the fuel over all of them, then divide once rather than averaging the separate mpg figures. If the result drops suddenly and stays low, check tire pressures and consider a service; the fuel cost calculator shows what the drop costs on a trip.
Frequently asked questions
How do I calculate my gas mileage?
Fill the tank and note the odometer. When you next fill up, subtract the old reading from the new one to get the miles driven, then divide by the gallons it took to fill the tank. 350 miles on 12.5 gallons is 28 mpg.
Why is my real mpg lower than the EPA rating?
EPA ratings come from standardized lab tests, while your figure reflects your own speed, traffic, climate, load and trip lengths. Short trips in cold weather, highway speeds well above 60 mph and idling all lower real-world economy.
What is a good gas mileage?
It depends on the vehicle. As a rough guide, many midsize sedans are rated around 28 to 35 mpg combined, small SUVs in the mid to high 20s, full-size pickups around 20 and hybrids 40 or more. Compare with the official rating for your model on fueleconomy.gov rather than with other types of vehicles.
Can I use the trip computer instead?
Trip computers are convenient, but they estimate fuel use rather than measure it, so they can be off by a few percent. The fill-up method measures the real fuel added, so it's the better check. Comparing the two over a few tanks tells you how far off your car's display is.
Related calculators
- Fuel Cost Calculator — The fuel cost of a road trip from distance, fuel economy and price, with round trips and cost per person.
- Mileage Reimbursement Calculator — Mileage reimbursement at the 2026 IRS rates for business, medical and charity driving, or your own rate per mile.
- Unit Conversion Calculator — Convert length, weight, temperature, area, volume, speed, time, data, pressure, energy, power and fuel economy.
- Tire Size Calculator — Compare two tire sizes: diameter, sidewall, circumference, revolutions per mile and the speedometer difference.
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