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Formula
MPG = Miles driven ÷ Gallons used
- Miles driven
- distance since the last fill-up (trip meter or odometer difference)
- Gallons used
- US gallons it took to fill the tank back up
What the MPG calculator does
Miles per gallon is the standard US measure of fuel economy: how far your car travels on one gallon of gas. The MPG calculator has two jobs. In From a fill-up mode it works out the mileage you actually got from the distance you drove and the fuel you put back in. In Convert units mode it turns any fuel economy figure into mpg, L/100 km, km/L or UK mpg, which helps when you’re reading a European review or an imported car’s spec sheet.
How to measure your gas mileage
The fill-up (or tank-to-tank) method is the most reliable way to find your real mpg:
- Fill the tank until the pump clicks off, and reset the trip meter (or write down the odometer reading).
- Drive as you normally would. A full tank, or at least a few hundred miles, gives a more representative number.
- Fill up again at the same pump if you can, stopping at the first click.
- Enter the miles on the trip meter and the gallons on the pump receipt into the MPG calculator.
If you use kilometers and liters, choose those units and the result switches to L/100 km, with mpg shown underneath. You can also mix units, such as miles with liters.
How to calculate mpg by hand
The formula is a single division: MPG = Miles driven ÷ Gallons used.
Suppose the trip meter reads 287.4 miles and the pump takes 9.82 gallons. 287.4 ÷ 9.82 = 29.27 mpg.
In metric, fuel consumption is usually written as liters per 100 km: L/100 km = Liters used ÷ Kilometers driven × 100. For 612 km on 45.3 liters, that’s 45.3 ÷ 612 × 100 = 7.4 L/100 km. Kilometers per liter is the plain division the other way round: 612 ÷ 45.3 = 13.51 km/L.
Converting mpg to L/100 km and back
Mpg tells you distance per fuel, while L/100 km tells you fuel per distance, so one is the reciprocal of the other. Because a US gallon is 3.785411784 liters and a mile is 1.609344 km, the conversion constant is 100 × 3.785411784 ÷ 1.609344 ≈ 235.215:
- L/100 km = 235.215 ÷ mpg. 34 mpg is 235.215 ÷ 34 = 6.92 L/100 km.
- mpg = 235.215 ÷ L/100 km. 8.4 L/100 km is 235.215 ÷ 8.4 = 28 mpg.
For UK mpg the constant is 282.481, because the imperial gallon is larger. To go from UK to US mpg, multiply by 0.8327: 42 UK mpg is about 34.97 US mpg.
Fuel economy conversion table
| L/100 km | US mpg | km/L | UK mpg |
|---|---|---|---|
| 4 | 58.8 | 25 | 70.6 |
| 5 | 47 | 20 | 56.5 |
| 6 | 39.2 | 16.67 | 47.1 |
| 7 | 33.6 | 14.29 | 40.4 |
| 8 | 29.4 | 12.5 | 35.3 |
| 9 | 26.1 | 11.11 | 31.4 |
| 10 | 23.5 | 10 | 28.2 |
| 12 | 19.6 | 8.33 | 23.5 |
| 15 | 15.7 | 6.67 | 18.8 |
Why mpg can be misleading
Mpg feels intuitive, but it hides how much fuel you really save. Improving a vehicle from 10 to 20 mpg saves 5 gallons every 100 miles. Improving another from 30 to 60 mpg saves only about 1.7 gallons over the same distance. Fueleconomy.gov calls this the “MPG illusion” and suggests thinking in fuel consumption, such as gallons per 100 miles, which the MPG calculator lists below the result. When you’re choosing between cars, compare fuel used, not just mpg.
Real-life uses
Tracking your car’s health
A sudden drop in mpg, with no change in driving or weather, can point to underinflated tires, a dragging brake or an engine problem. Logging each fill-up and running it through an mpg calculator makes the trend easy to spot.
Budgeting fuel
Once you know your real mpg, plug it into the gas cost calculator to price any drive, or into the trip cost calculator to budget a road trip with tolls, lodging and food.
Reading foreign specs
European and Canadian sources quote L/100 km, while UK sources quote imperial mpg. Use the conversion mode so you’re comparing like with like. To convert other volumes, such as liters to gallons, use the volume converter.
Common mistakes
- Using a partial fill. If you don’t fill up to the same point both times, the gallons figure is wrong. One odd reading isn’t a problem; average three or four.
- Forgetting to reset the trip meter. Without it, you’ll need the odometer difference between the two fill-ups.
- Mixing up US and UK gallons. UK figures look about 20% better for the same car.
- Treating L/100 km like mpg. With L/100 km, lower is better. A car using 5 L/100 km is more efficient than one using 8.
- Judging from one short trip. Cold starts and short errands use more fuel per mile, so a single short tank understates your normal mileage.
Glossary
- Mpg (US): miles per 3.785-liter US gallon.
- Mpg (UK): miles per 4.546-liter imperial gallon.
- L/100 km: liters burned per 100 kilometers. Lower is better.
- km/L: kilometers per liter, common in Asia and Latin America. Higher is better.
- Combined rating: the EPA’s weighted mix of city and highway driving.
Worked examples
Fuel economy from a fill-up
342 miles on the trip meter, then 12.6 gallons to fill the tank again.
Inputs (From a fill-up): Distance driven = 342; Distance driven unit = Miles; Fuel used = 12.6; Fuel used unit = Gallons (US)
MPG = 342 mi ÷ 12.6 gal = 27.14 mpg (US)
Answer: 342 mi on 12.6 gal is 27.14 mpg (US) (8.67 L/100 km).
Metric fill-up
615 km driven on 41.2 liters of fuel.
Inputs (From a fill-up): Distance driven = 615; Distance driven unit = Kilometers; Fuel used = 41.2; Fuel used unit = Liters
L/100 km = 41.2 L ÷ 615 km × 100 = 6.7 L/100 km
Answer: 615 km on 41.2 L is 6.7 L/100 km (35.11 mpg (US)).
Converting a European rating to mpg
A car rated at 5.2 L/100 km, converted to US miles per gallon.
Inputs (Convert units): Fuel economy = 5.2; From = Liters per 100 km (L/100 km); To = Miles per gallon, US (mpg)
mpg = 235.2146 ÷ 5.2 L/100 km = 45.23 mpg (US)
Answer: 5.2 L/100 km = 45.23 mpg (US).
UK mpg to US mpg
A UK figure of 50 mpg uses the larger imperial gallon, so it is fewer US miles per gallon.
Inputs (Convert units): Fuel economy = 50; From = Miles per gallon, UK (imperial); To = Miles per gallon, US (mpg)
50 mpg (UK) = 17.7003 km/L = 41.63 mpg (US)
Answer: 50 mpg (UK) = 41.63 mpg (US).
Assumptions and limitations
- Mpg is US miles per US gallon (3.785411784 L). UK mpg uses the imperial gallon of exactly 4.54609 L, so the same car shows a higher UK figure.
- L/100 km = 100 ÷ km/L, and mpg × L/100 km ≈ 235.215. Every conversion uses exact unit definitions (1 mile = 1.609344 km).
- In fill-up mode the result is shown in mpg when you enter miles and in L/100 km when you enter kilometers. Every other unit is listed below the result.
- The fill-up method assumes the tank is filled to the same level both times. A pump that clicks off early or late shifts a single reading, so average several fill-ups for a reliable figure.
- Results are rounded to 2 decimal places.
Frequently asked questions
How do you calculate mpg?
Fill the tank, reset the trip meter, drive normally, then fill up again. Divide the miles on the trip meter by the gallons it took to refill. 290 miles on 10 gallons is 29 mpg.
How do I convert mpg to L/100 km?
Divide 235.215 by the mpg figure. 30 mpg is 235.215 ÷ 30 = 7.84 L/100 km. The same division works the other way, so 7.84 L/100 km is about 30 mpg.
Why is UK mpg higher than US mpg for the same car?
A UK (imperial) gallon is 4.54609 liters, about 20% more than the 3.785-liter US gallon. More fuel per gallon means more miles per gallon, so 40 US mpg is about 48 UK mpg.
What is good gas mileage?
It depends on the vehicle class. Many compact gas cars average around 30 to 35 mpg combined, hybrids often exceed 45 mpg, and large pickups and SUVs are commonly under 25 mpg. Compare your result with the EPA rating for your model on fueleconomy.gov.
Why is my real mpg lower than the sticker rating?
EPA ratings come from standardized tests and are meant for comparing vehicles. Fueleconomy.gov explains that real mileage varies with how and where you drive, vehicle condition and maintenance, fuel variations, differences between individual cars and engine break-in, plus cold weather.
How do you calculate mpg for a hybrid?
The same way as any gas car. Fill the tank, drive, refill, and divide the miles by the gallons. A regular hybrid gets all its energy from gas, so the result is directly comparable. Plug-in hybrids that also charge from the grid are rated in MPGe, which counts electricity too, so fill-up mpg alone overstates their efficiency.
Can I calculate mpg without filling the tank?
Only roughly. The car's trip computer estimate is convenient but can be off by a few percent. The fill-up method measures the fuel that actually went in, which is why it's the standard way to check mileage.
Sources
- Your Mileage Will Vary — U.S. Department of Energy and EPA (fueleconomy.gov). Checked October 10, 2026.
- Learn More About the Fuel Economy Label for Gasoline Vehicles — U.S. Department of Energy and EPA (fueleconomy.gov). Checked October 10, 2026.
- Weights and Measures Act 1985, Schedule 1 (definition of the gallon) — UK legislation.gov.uk. Checked October 10, 2026.
- Approximate Conversions from U.S. Customary Measures to Metric — National Institute of Standards and Technology. Checked October 10, 2026.