Mileage / Fuel Economy Calculator

Measure your car's real fuel economy from a distance driven and the fuel it used, and see it in every unit — mpg (US and UK), km per litre, and L/100km — so you can compare figures from any source.

How to use this calculator

Enter the distance driven since the last fill (km or miles).
Enter the fuel used to refill — the pump total works perfectly.
Read your economy in every unit at once.

Formula used

L/100km = Litres ÷ km × 100 · mpg = miles ÷ gallons

Fill the tank, note the odometer, drive, refill and note the litres and new distance — that fuel exactly refilled what you burned over that distance. Watch the gallon: US gallons (3.785 L) and UK gallons (4.546 L) differ by ~20%, so their mpg figures aren't comparable.

Example calculation

Worked example

500 km on 37.5 litres: 37.5 ÷ 500 × 100 = 7.5 L/100km = 13.3 km/L = 31.4 mpg (US) = 37.7 mpg (UK).

The same car scores differently in each unit — which is why quoting the unit matters.

One economy, many units

Fuel economy is measured in mutually confusing ways around the world, and the difference isn't cosmetic. L/100km and km/L are inverses (lower L/100km is better; higher km/L is better), which alone trips people up. On top of that, US and UK gallons differ by about 20%, so a car rated "40 mpg" in Britain is really about 33 mpg in American terms — the same fuel burn, a very different-looking number.

The reliable way to know your real economy is the brim-to-brim method: fill up, zero the trip meter, drive normally, then refill and divide. That captures actual conditions, unlike the optimistic factory figure. This calculator converts a single real measurement into every unit so you can compare against any brochure, forum post or friend's car regardless of which convention they use.

Why use this calculator?

Convert between mpg (US/UK), km/L and L/100km without confusion.
Measure real economy from a brim-to-brim fill.
Compare cars and figures quoted in different countries' units.

Frequently asked questions

How do I calculate my car's fuel economy?

Fill the tank, reset the trip meter, drive, then refill. Divide the litres by the km and multiply by 100 for L/100km, or divide miles by gallons for mpg. The refill amount is exactly the fuel you used over that distance.

Why is UK mpg higher than US mpg for the same car?

Because a UK gallon (4.546 L) is about 20% larger than a US gallon (3.785 L). More litres per gallon means more miles per gallon for identical real economy. L/100km avoids this ambiguity entirely.

Is a lower or higher number better?

It depends on the unit. For L/100km, lower is better (less fuel per distance). For mpg and km/L, higher is better (more distance per fuel). They measure the same thing from opposite directions.