How this calculator works
Fuel economy compares distance travelled with fuel used. Miles per gallon and kilometres per litre rise as a vehicle becomes more efficient. Litres per 100 kilometres falls as efficiency improves, because it measures fuel used for a fixed distance. That reciprocal relationship is why doubling mpg does not double L/100 km: it halves it. The US and imperial gallons are different sizes, so US mpg and UK mpg are not interchangeable even though both are called mpg.
Formula
If the input is not L/100 km, km/L = value x F(unit), where F is 1 for km/L, 1.609344 / 3.785411784 for US mpg, and 1.609344 / 4.54609 for imperial mpg. If the input is L/100 km, km/L = 100 / value. Then L/100 km = 100 / (km/L), US mpg = (km/L) / 0.4251437 and UK mpg = (km/L) / 0.3540062.
Worked example
30 mpg (US) is 7.8405 L/100 km, 12.754 km/L and 36.028 mpg (UK). 5 L/100 km is 20 km/L, 47.043 mpg (US) and 56.496 mpg (UK). 30 mpg (UK) is 9.416 L/100 km.
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Frequently asked questions
Why is L/100 km inverted compared with mpg?
MPG counts distance per unit of fuel, while L/100 km counts fuel per fixed distance. A better car travels further on a gallon but uses fewer litres per 100 km, so the two numbers move in opposite directions.
Is a UK gallon the same as a US gallon?
No. The imperial gallon is 4.54609 litres and the US gallon is 3.785411784 litres, so 30 mpg (UK) is about 25 mpg (US).
Can I convert 0 mpg to L/100 km?
No. Zero fuel economy would mean infinite fuel use, and the reciprocal is undefined. This calculator requires a value greater than zero.