MPG Calculator
Turn miles and gallons into MPG, L/100 km, fuel cost and cost per mile.
Free, no sign-upIncludes a step-by-step guideBy AAutomotives Editorial Team
MPG and fuel cost
Miles per gallon–
L/100 km–
Fuel cost–
Cost per mile–
Knowing your real fuel economy is the first step to lowering fuel cost. Fill the tank, drive normally, fill it again, and enter the miles and gallons below. For trip planning with a known distance, use our fuel cost calculator.
How to measure MPG properly
- Fill the tank to the automatic shut-off and note the odometer or reset the trip meter.
- Drive as normal until the tank is at least half empty.
- Fill up again at the same pump, to the same shut-off point, and note the gallons added.
- Divide miles driven by gallons added. Repeat for several tanks and average the result.
Formulas
MPG = miles ÷ gallons.
Cost per mile = price per gallon ÷ MPG.
L/100 km = 235.215 ÷ MPG (US gallons).
MPG = miles ÷ gallons.
Cost per mile = price per gallon ÷ MPG.
L/100 km = 235.215 ÷ MPG (US gallons).
Worked example
You drive 300 miles and add 12 gallons. MPG = 300 ÷ 12 = 25. At $3.50 per gallon the fuel cost is 12 × 3.50 = $42.00, which is $0.14 per mile. In metric terms, 235.215 ÷ 25 = 9.4 L/100 km.
Why your number differs from the sticker
- Short trips and cold starts burn more fuel than steady highway driving.
- High speed, hard acceleration and heavy braking reduce economy.
- Low tire pressure, roof racks and extra weight add drag and rolling resistance.
- Winter fuel blends and cold air lower the figure in winter.
- Using the onboard trip computer only is less accurate than tank-to-tank maths.
Common mistakes
- Using a single tank. One partial fill can skew the result, so average several.
- Mixing US and imperial gallons. An imperial gallon is larger, so imperial MPG figures look higher.
- Not filling to the same shut-off point each time.
- Comparing city-only and highway-only numbers as if they were the same trip.
Frequently asked questions
How do I calculate MPG?
Divide the miles driven by the gallons of fuel used between two full fill-ups.
What is a good MPG?
It depends on the vehicle type. Compare against the same class of vehicle and against your own history.
How do I convert MPG to L/100 km?
Divide 235.215 by the US MPG figure. The scales run in opposite directions, so a higher MPG means a lower L/100 km.
Does tire pressure affect MPG?
Yes. Under-inflated tires have more rolling resistance and can lower fuel economy.