How to Convert MPG to L/100km (and Back)

To convert miles per gallon to litres per 100 km, divide a constant by the mpg figure: 235.2 for US mpg, or 282.5 for UK mpg. So 30 US mpg is 235.2 ÷ 30 = about 7.84 L/100km. The fuel economy converter does it instantly across all four measures.

That is the rule. The interesting part is why it is a division rather than a simple multiply.

Why the conversion is inverse

Most conversions are a straight factor: multiply to go one way, divide to go back. Fuel economy is different because the two common units measure opposite things.

Miles per gallon is distance per unit of fuel. More is better.
Litres per 100 km is fuel per unit of distance. Less is better.

One is the reciprocal of the other, so converting between them means dividing into a constant rather than multiplying by one. That is also why a car with great economy has a high mpg but a low L/100km, which can feel back-to-front until you see why.

How to convert fuel economy

US mpg to L/100km

Divide 235.214 by the mpg.

25 mpg → 9.4 L/100km
40 mpg → 5.9 L/100km
50 mpg → 4.7 L/100km

L/100km to US mpg

Divide 235.214 by the L/100km (same constant, both ways).

5 L/100km → 47 mpg
8 L/100km → 29.4 mpg

The fuel economy converter also handles UK mpg and kilometres per litre.

US versus UK mpg

This trips people up constantly. A US gallon (3.785 litres) is smaller than a UK gallon (4.546 litres), so the same car covers fewer miles on a US gallon and shows a lower mpg. A car advertised at 48 mpg in Britain is only about 40 mpg in American terms. When comparing cars across the two markets, convert both to L/100km first, since that unit is the same everywhere and removes the gallon ambiguity entirely.

When it matters

This conversion comes up when comparing cars sold in different markets, driving a hire car abroad with an unfamiliar fuel sticker, or tracking your own consumption in a unit your records do not use. Because the relationship is inverse, eyeballing it is error-prone, so the converter is worth using rather than guessing.

To convert speeds between systems, see the speed converter.

Frequently asked questions

How do I convert MPG to L/100km?

Divide a constant by the mpg figure: about 235.2 for US mpg, or 282.5 for UK mpg. So 40 US mpg is 235.2 ÷ 40 = about 5.9 L/100km. It is a division into a constant, not a multiplication, because the two units measure opposite things.

Why is a lower L/100km better but a higher MPG better?

Because they measure opposite things. Miles per gallon is distance per fuel, so more is better. Litres per 100 km is fuel per distance, so less is better. That is also why converting between them is an inverse relationship rather than a straight factor.

Is US mpg the same as UK mpg?

No. They use different gallons. A UK gallon is larger, so the same car shows a higher number in UK mpg than in US mpg. A car rated 48 UK mpg is only about 40 US mpg.