MPG calculator
MPG is miles driven divided by gallons used. If you drove 312 miles and refilled the tank with 10.4 gallons, your car got 30.0 MPG, which is 7.8 L/100 km.
On this page (8 sections)
How to use the calculator
- Miles. Enter the miles from your trip odometer, or the odometer reading at the last fill-up and today’s reading.
- Gallons. Enter the gallons you just pumped to fill the tank back up. The pump and the receipt show it.
- More fill-ups (optional). Add rows for each tank to get your average MPG.
- Compare (optional). Pick your car to see its EPA rating next to your result.
For the most accurate number, fill the tank all the way at both fill-ups. The full method, step by step, is in how to calculate MPG.
The formula
MPG = miles driven ÷ gallons used.
Example: the odometer read 45,210 at the last full tank and 45,522 today. 45,522 − 45,210 = 312 miles. You pumped 10.4 gallons to fill up again. 312 ÷ 10.4 = 30.0 MPG.
This is the method fueleconomy.gov describes: fill the tank, write down the odometer, fill up again at the next stop, and divide the miles by the gallons it took.
Several fill-ups: average MPG the right way
To average several tanks, add up all the miles and all the gallons, then divide. Don’t average the MPG numbers themselves: a short tank would count as much as a long one.
| Tank | Miles | Gallons | MPG |
|---|---|---|---|
| 1 (highway) | 300 | 10 | 30.0 |
| 2 (city) | 150 | 6 | 25.0 |
| Total | 450 | 16 | 28.1 |
Right: 450 ÷ 16 = 28.1 MPG. Wrong: (30.0 + 25.0) ÷ 2 = 27.5 MPG.
MPG and L/100 km
Most of the world measures fuel use as liters per 100 kilometers (L/100 km). Lower is better, the opposite of MPG. To convert, divide 235.215 by the number: 235.215 ÷ 30 MPG = 7.8 L/100 km, and 235.215 ÷ 7.8 L/100 km = 30.2 MPG (the small gap is rounding).
| MPG | 20 | 25 | 30 | 35 | 40 | 50 |
|---|---|---|---|---|---|---|
| L/100 km | 11.8 | 9.4 | 7.8 | 6.7 | 5.9 | 4.7 |
U.S. gallons. A UK (imperial) gallon is larger, so UK MPG figures are higher for the same car.
Your MPG vs the EPA rating
The EPA rating on the window sticker comes from lab tests. The car runs on a dynamometer (a machine like a treadmill) through set routines that represent city and highway driving, according to fueleconomy.gov. Every car takes the same tests, so the ratings are fair for comparing cars. But no test fits every driver, road, and season, and your MPG can come out higher or lower.
What usually pulls real MPG down: aggressive driving (roughly 15%–30% lower at highway speeds), the AC on max (roughly 5%–25%), and cold weather (about 15% lower in city driving at 20 °F than at 77 °F), all per fueleconomy.gov. Under-inflated tires cost about 0.2% for every 1 psi drop. A new engine can take 3,000–5,000 miles to reach its best mileage.
A sudden drop with no change in weather or driving is worth a look: check tire pressure first. Our car maintenance checklist lists the basics.
Common mistakes
- A partial fill-up. If the tank was not full, the gallons don’t match the miles. Fill all the way at both ends, or average over more tanks.
- A missed fill-up. If you skip writing down one fill-up, the next result looks far too high.
- Mixing up the trip odometer. Reset it at each full tank, or use the main odometer.
- Trusting one tank. One tank can be misleading if the pump clicked off early. Use the average of several.
Once you know your MPG, you can price a drive with the gas trip calculator or see your fuel cost per mile.
FAQ
How many fill-ups do I need for an accurate MPG?
One full-to-full tank gives a fair number. Three or more tanks, added together, smooth out pumps that click off early and changes in weather and driving.
How do I convert MPG to L/100 km?
Divide 235.215 by the MPG. 30 MPG is 7.8 L/100 km. The same formula works the other way: 235.215 ÷ L/100 km = MPG.
Why is my MPG lower than the EPA estimate?
The EPA rating comes from standard lab tests. Real driving adds things the test can’t match exactly: higher speeds, hard acceleration, cold weather, short trips, AC, heavy loads, and low tire pressure. So your number can land below the rating, or above it.
Can I use this calculator for a hybrid or a diesel?
Yes. The math is the same for any car that runs on fuel you buy by the gallon: miles ÷ gallons. For a plug-in hybrid, the result leaves out the electricity you charged, so it is not a full picture of energy use.
Sources
- fueleconomy.gov (U.S. DOE and EPA), How to calculate your MPG (checked November 12, 2026): fueleconomy.gov
- fueleconomy.gov, How vehicles are tested: fueleconomy.gov
- fueleconomy.gov, Factors that affect MPG (driving, AC, engine break-in): fueleconomy.gov
- fueleconomy.gov, Keeping your vehicle in shape (tire pressure): fueleconomy.gov
- fueleconomy.gov, Fuel economy in cold weather: fueleconomy.gov