Calculate your vehicle's fuel economy in four common formats, US MPG, UK/Imperial MPG, liters per 100km, and km per liter, from the distance you drove and the fuel you used.
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The US and UK both use miles per gallon, but a US gallon (3.785 liters) is smaller than a UK/Imperial gallon (4.546 liters), so the same vehicle shows a higher MPG figure in the UK than in the US. Most of the rest of the world uses liters per 100km instead, where a lower number means better efficiency, the opposite direction from MPG. This calculator converts your trip data into all four formats at once so you can compare figures regardless of which system a source uses.
Our Fuel Cost Calculator estimates how much a trip will cost given a fuel efficiency rating and fuel price you already know. This calculator does the opposite, it derives your actual fuel economy from a real trip's distance and fuel used, and expresses it across every common unit system, useful for verifying your vehicle's real-world efficiency or comparing it against another vehicle's rating listed in a different unit.
Driving 300 miles and using 12 US gallons of fuel: US MPG = 300 ÷ 12 = 25 MPG. The same trip expressed in other systems comes out to about 30.02 MPG (UK/Imperial, since a UK gallon is larger), 9.41 L/100km, and 10.63 km/L, all four describing the exact same underlying fuel efficiency, just in the units common to different countries.
Comparing a US MPG figure directly against a UK-sourced MPG rating without converting first, a US gallon is about 17% smaller than a UK/Imperial gallon, so the same vehicle shows meaningfully different MPG numbers between the two systems, this tool converts both automatically so you're always comparing like with like.
Not filling the tank completely at both the start and end of the measurement period, a partial fill-up understates the actual fuel used for the distance driven, skewing the calculated economy in either direction.
Forgetting that L/100km runs in the opposite direction from MPG, a lower L/100km number means better efficiency, while a lower MPG means worse efficiency, misreading which direction is 'better' for L/100km is an easy mistake when switching between unit systems.
Official ratings come from standardized lab tests. Real-world fuel economy varies with driving style, terrain, weather, tire pressure and load, so calculating from an actual fill-up, as this tool does, usually gives a more accurate picture of your everyday efficiency.
For MPG (US or UK) and km/L, higher is better, more distance per unit of fuel. For L/100km, lower is better, less fuel needed to cover 100km.
Fill your tank completely, reset your trip odometer, drive normally, then fill up completely again and note the distance driven and the amount of fuel needed to refill, that's your fuel used for the trip.
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