How to convert mpg to litres per 100 km (and back)
Convert fuel economy between US mpg, UK mpg, litres per 100 km and km per litre, with tables, formulas, and why US and UK figures differ for the same car.
Published
Tools for this job: Volume Converter · Length Converter
A car review from the US says 30 mpg, a British one says 36 mpg, and the European brochure says 7.8 litres per 100 km. They may all describe the same car. Fuel economy units differ in two ways: the gallon itself is a different size in the US and UK, and Europe measures fuel used per distance rather than distance per fuel.
Conversion table
Litres per 100 km for each mpg figure, using US gallons and UK gallons:
| mpg | US | UK |
|---|---|---|
| 20 | 11.8 | 14.1 |
| 25 | 9.4 | 11.3 |
| 30 | 7.8 | 9.4 |
| 40 | 5.9 | 7.1 |
| 50 | 4.7 | 5.6 |
The formulas
- L/100 km = 235.21 ÷ US mpg
- L/100 km = 282.48 ÷ UK mpg
- The same formulas work in reverse: US mpg = 235.21 ÷ L/100 km.
- UK mpg = US mpg × 1.201
- km per litre = 100 ÷ L/100 km
Example: a car using 7 litres per 100 km does 235.21 ÷ 7 = 33.6 US mpg, or 40.4 UK mpg, or 14.3 km per litre.
Why the US and UK numbers differ
A US gallon is 3.785 litres. A UK (imperial) gallon is 4.546 litres, about 20% bigger. A car goes further on the bigger gallon, so its UK mpg figure is about 20% higher than its US one. Neither car is more efficient; the units are different. The Volume Converter keeps US and imperial gallons separate for this reason.
Why L/100 km runs backwards
Miles per gallon is distance per fuel, so higher is better. Litres per 100 km is fuel per distance, so lower is better. The L/100 km figure is more useful for working out costs: if you drive 15,000 km a year at 7 L/100 km, you use 150 × 7 = 1,050 litres, and multiplying by the price per litre gives the annual fuel bill.
It also shows real savings more honestly. Going from 15 to 20 mpg saves more fuel over the same distance than going from 40 to 50 mpg, even though the second change looks bigger.
km per litre
India and some other countries quote kilometres per litre. 15 km/L is 6.7 L/100 km, about 35 US mpg or 42 UK mpg. Higher is better, like mpg.
Working out your own figure
Fill the tank to the brim, reset the trip counter, and drive normally. At the next fill-up, fill to the brim again and note the litres or gallons added. Divide the distance by the fuel for km per litre or mpg, or divide the fuel by the distance and multiply by 100 for L/100 km. Averaging over two or three tanks evens out how full the pump left it.
Official versus real-world figures
Brochure figures come from standardised laboratory tests and are usually better than real-world driving, especially in cold weather, short trips and at motorway speeds. Compare cars using the same test, and expect your own figure to be somewhat worse.
For distance conversions between miles and kilometres, use the Length Converter. For electric cars, see how much it costs to charge an electric car.