MPG = miles driven ÷ gallons used. Fill the tank, reset the trip odometer, drive, then fill up again: the gallons it takes to refill is the fuel you used. 320 miles on 11.2 gallons is 28.57 mpg.
Metric countries quote fuel consumption the other way round, in liters per 100 km, where lower is better. Because a U.S. gallon is 3.785412 L and a mile is 1.609344 km, L/100 km = 235.21 ÷ mpg, so 28.57 mpg is 8.23 L/100 km. The calculator also gives km/L and U.K. (imperial-gallon) mpg.
How to use the mpg calculator (gas mileage)
- Fill the tank completely and reset the trip meter.
- Drive normally, then fill up again and note the gallons (or liters) and the miles (or km).
- Enter the numbers. Optionally add the price per gallon for cost per mile.
- Average a few tanks for a steadier figure.
Formula
The constants come from the exact definitions: 100 × 3.785411784 L ÷ 1.609344 km = 235.2146; an imperial gallon (4.54609 L) is 1.20095 U.S. gallons.
Worked example
You drove 320 miles and refilled with 11.2 gallons at $3.50.
- 320 ÷ 11.2 = 28.57 mpg.
- 235.2146 ÷ 28.57 = 8.23 L/100 km (12.15 km/L; 34.31 UK mpg).
- Cost per mile: $3.50 ÷ 28.57 = $0.122.
MPG vs L/100 km
| US mpg | L/100 km | km/L |
|---|---|---|
| 15 | 15.68 | 6.38 |
| 20 | 11.76 | 8.50 |
| 25 | 9.41 | 10.63 |
| 30 | 7.84 | 12.75 |
| 40 | 5.88 | 17.01 |
| 50 | 4.70 | 21.26 |
Because MPG is "distance per fuel," gains at the low end matter most: going from 15 to 20 mpg saves more fuel over 100 miles (1.67 gal) than going from 40 to 50 mpg (0.5 gal). To price a trip, use the fuel cost calculator (gas trip cost).
Getting an accurate number
The "fill-to-fill" method is the most reliable way to measure fuel economy at home:
- Fill the tank until the pump clicks off the first time. Don't top off.
- Reset the trip odometer (or write down the odometer reading).
- Drive as usual until you need fuel, ideally at least half a tank.
- Fill again at the same pump if you can, stopping at the first click, and note the gallons and the miles.
- Repeat for three or more tanks and add up the totals: total miles ÷ total gallons smooths out filling differences.
The car's dashboard readout is a useful guide, but it is an estimate and can differ from fill-to-fill results.
Using your MPG
Once you know your real MPG, you can budget trips and compare cars. Divide any distance by your MPG for gallons needed, and multiply by the price per gallon for cost; the fuel cost calculator (gas trip cost) does both. When comparing two vehicles, compare fuel used per distance rather than MPG differences: at 12,000 miles a year, a 20 mpg car uses 600 gallons and a 25 mpg car uses 480, a 120-gallon difference, while going from 40 to 50 mpg saves only 60 gallons.
Caveats and assumptions
- Accuracy depends on filling to the same level each time; average several tanks.
- Odometer error of a few percent carries straight into MPG.
Frequently asked questions
How do I calculate MPG?
Divide the miles driven since the last full tank by the gallons needed to refill. 320 mi ÷ 11.2 gal = 28.57 mpg.
How do I convert MPG to L/100 km?
Divide 235.21 by the MPG. 30 mpg = 7.84 L/100 km.
How do I convert L/100 km to MPG?
Divide 235.21 by the L/100 km figure. 6 L/100 km = 39.2 mpg.
Why is UK mpg higher than US mpg?
The imperial gallon is about 20% larger (4.546 L vs 3.785 L), so the same car goes further per UK gallon.
Is my dashboard MPG accurate?
It is an estimate. Measuring fill-to-fill over several tanks gives a more reliable figure.
Sources
- NIST SP 811 Appendix B.9 — Conversion factors — US and imperial gallon, mile, mpg factors
- FuelEconomy.gov — Learn More About the Fuel Economy Label — how official MPG figures are presented
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.