Fuel Economy Converter
Convert litres per 100 km to mpg — and between UK and US mpg, which use different gallons — plus kilometres per litre. Results update as you type; exact factors shown.
One miles per imperial gallon is 282.4809 litres per 100 km — see the exact factor, not a rounded rule of thumb.
How the fuel economy converter works
Fuel economy is an inverse quantity: lower L/100 km is better, higher mpg is better. mpg (US) = 235.215 / (L/100 km) because a US gallon is 3.785 L and a mile 1.609 km; mpg (UK) = 282.481 / (L/100 km) with the 4.546 L imperial gallon. UK mpg figures are therefore always 20% higher than US mpg for the same car.
The UK is metric for most trade and science but keeps miles, pints, stone and mpg (with the imperial gallon) in daily life.
Common fuel economy conversions
| From | To | Units |
|---|---|---|
| 7 L/100 km | 33.6021 mpg (US) | litres per 100 km to miles per US gallon |
| 7 L/100 km | 40.3544 mpg (UK) | litres per 100 km to miles per imperial gallon |
| 30 mpg (US) | 7.8405 L/100 km | miles per US gallon to litres per 100 km |
| 40 mpg (UK) | 7.062 L/100 km | miles per imperial gallon to litres per 100 km |
| 5 L/100 km | 20 km/L | litres per 100 km to kilometres per litre |
| 15 km/L | 6.6667 L/100 km | kilometres per litre to litres per 100 km |
| 10 L/100 km | 23.5215 mpg (US) | litres per 100 km to miles per US gallon |
| 50 mpg (UK) | 41.6337 mpg (US) | miles per imperial gallon to miles per US gallon |
| 25 mpg (US) | 30.0237 mpg (UK) | miles per US gallon to miles per imperial gallon |
| 4.5 L/100 km | 62.7735 mpg (UK) | litres per 100 km to miles per imperial gallon |
Units in this converter and their exact factors
| Unit | Symbol | To L/100 km |
|---|---|---|
| litres per 100 km | L/100 km | x |
| kilometres per litre | km/L | 100÷x |
| miles per imperial gallon | mpg (UK) | 282.480936÷x |
| miles per US gallon | mpg (US) | 235.214583÷x |
FAQFrequently asked questions about fuel economy conversion
Why is UK mpg higher than US mpg for the same car?
An imperial gallon is 4.546 L, a US gallon 3.785 L — 20% more fuel per gallon, so 20% more miles per gallon.
What is 7 L/100 km in mpg?
33.6 mpg (US) or 40.4 mpg (UK).
How do I convert mpg to L/100 km?
Divide 235.215 by US mpg, or 282.481 by UK mpg.
Is a lower L/100 km better?
Yes — it is fuel used per distance, so lower means more economical. With mpg, higher is better.
What is good fuel economy?
Roughly under 6 L/100 km (over 39 mpg US / 47 mpg UK) for a petrol car; hybrids reach 4 L/100 km; EVs are rated in kWh/100 km instead.
Where these figures come from
- BIPM — The International System of Units (SI Brochure, 9th edition) — definitions of the base and derived units.
- NIST Special Publication 811 — Guide for the Use of the International System of Units — the exact conversion factors used here.
- National Physical Laboratory (NPL) — United Kingdom's national measurement authority.
Last checked: September 2026. Conversion factors are fixed by definition and do not change with tax years or regulations.