Fuel Economy Calculator
Work out your real fuel economy from a single fill-up, or convert any figure between MPG and L/100km. Enter your numbers and the results — MPG (US and UK), litres per 100 km, and km per litre — update as you type. Nothing is uploaded.
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Enter your distance and fuel to see the economy.
How to use the fuel economy calculator
There are two ways to use this tool, chosen with the toggle at the top.
Measure from a fill-up. The most accurate method. Fill your tank, reset your trip meter, and drive as usual. At your next fill-up, enter the distance you covered and the amount of fuel it took to fill back up. Pick whether you measured in miles or kilometres and in US gallons, UK gallons, or litres. The calculator returns your economy in every common unit at once.
Convert units. Already know one figure — say 35 MPG — and want the metric equivalent? Switch to convert mode, type the value, and choose which unit it's in. You'll get the matching MPG (US and UK), L/100km, and km/L.
Everything runs in your browser — the numbers you type are never uploaded.
Understanding the units
Fuel economy is measured two opposite ways around the world, and it helps to know which is which:
- Miles per gallon (MPG): distance per unit of fuel. Higher is better. Used in the US and UK — but the two countries use different-sized gallons, so the same car shows different MPG numbers.
- Kilometres per litre (km/L): the metric version of the same idea. Higher is better.
- Litres per 100 km (L/100km): fuel needed to cover a fixed distance. Lower is better. The standard across most of Europe and the rest of the world.
Common fuel economy questions
How do I calculate my MPG? Divide the miles you drove by the gallons of fuel you used to refill. This tool does it for you and adds the metric equivalents.
What's the difference between US and UK MPG? A UK gallon (about 4.546 L) is bigger than a US gallon (about 3.785 L), so the same car always posts a higher number in UK MPG than in US MPG even though the real efficiency is the same.
How do I convert MPG to L/100km? Divide 235.21 by the US MPG figure. So 30 US MPG is about 7.84 L/100km. Convert mode does this for you in both directions.
Is a higher or lower number better? Higher MPG and km/L are better; lower L/100km is better. They all describe the same efficiency from different angles.
Why is my real MPG below the sticker? Official figures are lab numbers. Your driving style, speed, traffic, cold starts, short trips, tyre pressure, and load all push real-world economy lower — which is exactly why measuring it from a real fill-up is worth doing.
Results are estimates for general information. Measure across several fill-ups for the most reliable average.