Fuel Economy & Converter
Fuel economy from a fill-up (distance and fuel in any unit) and any figure converted between L/100 km, km/L, mpg (US) and mpg (UK) with exact factors.
One economy figure in all four units at once, from a trip or from a value you already have, with the US/UK gallon difference made explicit.
Example: 500 km on 32 L is 6.4 L/100 km = 15.6 km/L = 36.8 mpg (US) = 44.1 mpg (UK).
One tank,
four units.
How economy is measured from a fill-up, why the four units are the same number in different clothes, the exact gallons and miles, and what a single sample can and cannot say.
From a fill-up
Fill the tank, drive, fill it again: fuel economy is the distance driven divided by the fuel added. Distance may be in kilometres or miles and fuel in litres or either gallon; the page converts everything to litres per 100 km first and reports all four figures. Brim-to-brim fills over several tanks average out the pump cut-off, which alone can swing one tank by several percent.
The four units
L/100 km is consumption (lower is better); km/L and mpg are economy (higher is better); mpg (US) uses the 3.785411784 L gallon and mpg (UK) the 4.54609 L imperial gallon, so a UK figure is 20.1 % higher for the same car. A mile is exactly 1.609344 km. The familiar constant is 235.215: mpg (US) = 235.215 ÷ L/100 km.
Ratings and reality
Official figures (EPA, WLTP, NEDC) are test-cycle results and are not comparable with a measured trip; a trip computer estimates from injector time and can read optimistic. Weather, load, speed and short trips move real economy by far more than the difference between units.
What the number does not settle
What a trip costs is the next step — see Fuel & Trip Cost; the full cost of driving, with depreciation and insurance, is Cost per Mile. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
Last reviewed 18 September 2026. How results are checked: How we verify.