How to use the fuel economy converter
- Enter the figure in Value. The default is 6.5.
- In From, pick its unit: L/100 km, km/L, mpg (US), mpg (UK, imperial), miles per litre, L/100 miles or US gallons/100 miles.
- In To, pick the unit you want. The answer appears straight away.
- Check the table under the result for all seven units, or press swap to reverse the direction.
What it does and when to use it
Countries rate cars in different ways. Europe, Australia and Canada mostly use litres per 100 km. The US uses miles per US gallon, the UK uses miles per imperial gallon, and India and Japan often quote km per litre. Reading a review, a car advert or a rental agreement from another country means converting.
Use it to compare an imported car’s official rating with your current car, or to turn your trip computer’s figure into the unit a friend uses. Once you have a figure you trust, the fuel cost calculator turns it into money for a trip. For fuel volumes alone, such as gallons to litres, use the volume converter, and for distance the length converter.
How it works
The tool stores every unit as kilometres per litre (km/L).
- mpg (US) × 0.425143707 = km/L (1.609344 km ÷ 3.785411784 L)
- mpg (UK) × 0.354006189 = km/L (1.609344 km ÷ 4.54609 L)
- miles per litre × 1.609344 = km/L
- L/100 km = 100 ÷ km/L
- L/100 miles = 160.9344 ÷ km/L
- US gallons/100 miles = 160.9344 ÷ (km/L × 3.785411784)
The mile, the US gallon (231 cubic inches) and the imperial gallon are exact definitions, so the factors are exact too. NIST’s SI guide gives the same shortcut: divide 235.215 by US mpg to get L/100 km.
Worked examples
A US car rated 30 mpg. That is 7.84048611111 L/100 km, 12.7543112229 km/L, and 3.33333333333 US gallons per 100 miles.
The same car in UK units. 30 US mpg = 36.0284977651 mpg (UK, imperial). A British review of the same car would quote the higher number.
A zero entry. 0 km/L gives the message “Fuel economy must be greater than zero (0 would mean infinite consumption or no distance).”
Limits and tips
- Official ratings use test cycles (such as WLTP or the EPA cycles) that rarely match real driving. Converting the unit does not convert the test, so a WLTP figure and an EPA figure for the same car can still disagree.
- Electric cars use kWh per 100 km or miles per kWh, which this tool does not cover.
- Negative values are rejected for the same reason as zero.
Frequently asked questions
What is the quick formula from mpg to L/100 km?
Why is a UK mpg figure higher than a US one for the same car?
Why does L/100 km go down when the car gets better?
Why can't I enter 0?
Are fuel savings even across the mpg scale?
Sources
- NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity (fuel consumption) — NIST, accessed 2026-10-08
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