MPG vs L/100km: How to Compare Fuel Economy

To convert US miles per gallon to liters per 100 km, divide 235.215 by the mpg figure. For UK (imperial) mpg, divide 282.481 instead. So 30 US mpg is 7.84 L/100km, and the same car is 36 mpg in UK units. The two scales run in opposite directions: a higher mpg is better, but a lower L/100km is better.

The formulas and where the numbers come from

L/100km = 235.215 / US mpg        US mpg = 235.215 / L/100km
L/100km = 282.481 / UK mpg        UK mpg = 282.481 / L/100km
UK mpg  = US mpg × 1.20095
km/L    = 100 / L/100km

The constants come from the unit definitions. A mile is 1.609344 km, a US gallon is 3.785411784 liters and a UK gallon is 4.54609 liters. 100 × 3.785411784 ÷ 1.609344 = 235.2146, which rounds to 235.215. The UK version gives 282.4809. Because the formula divides, the conversion works the same way in both directions.

The fuel economy converter does this between US mpg, UK mpg, km/L and L/100km, and shows your value in every unit at once.

Conversion table

And from the European side, starting with L/100km:

A rough rule for mental math: L/100km is about 235 divided by US mpg, so 25 mpg is a bit under 10 L/100km, and 50 mpg is a bit under 5. For UK mpg, use 282.

Why L/100km is easier to compare

Miles per gallon measures distance per unit of fuel. L/100km measures fuel per unit of distance. When you want to know how much fuel you will save, the second one is the honest number.

Here is fuel used over 10,000 miles:

A 10 mpg gain on a thirsty car saves ten times more fuel than a 10 mpg gain on an efficient one. Most people expect the opposite. Researchers Richard Larrick and Jack Soll called this the “MPG illusion” in a 2008 paper in Science, and suggested labeling fuel use per distance instead.

With L/100km the math is linear. Going from 10 to 8 L/100km saves 300 liters over 15,000 km. Going from 6 to 4 L/100km also saves 300 liters. Same step, same saving.

Working out fuel for a trip

• In L/100km: liters = distance in km × L/100km ÷ 100. A 500 km trip at 6.5 L/100km uses 32.5 liters. At 1.80 per liter, that is 58.50 in fuel.
• In mpg: gallons = miles ÷ mpg. A 300-mile trip at 30 US mpg uses 10 gallons. At $3.20 a gallon, that is $32.00.

Rated figures come from lab tests. Real-world use is often worse in cold weather, at high speed and on short trips, so treat the rating as a best case.

Common mistakes

• Mixing US and UK gallons. A UK gallon is about 20% bigger, so the same car shows a higher mpg in UK units. 7 L/100km is 33.6 US mpg but 40.4 UK mpg. A UK review and a US review of the same car can be 7 mpg apart and both be right.
• Averaging mpg figures. Drive 100 miles at 20 mpg and 100 miles at 40 mpg, and your trip average is 26.7 mpg, not 30. You used 5 gallons plus 2.5 gallons for 200 miles. For legs of equal length you can average the L/100km values. Otherwise divide total distance by total fuel.
• Reading lower as worse. In L/100km a smaller number is the better car. 5 L/100km beats 7 L/100km.
• Comparing km/L with L/100km. They look similar but are inverses. 20 km/L is 5 L/100km.

More unit conversion guides and tools: free unit converters.

Frequently asked questions

How do I convert L/100km to mpg?

Divide 235.215 by the L/100km figure for US mpg, or 282.481 for UK mpg. 6 L/100km is 39.2 US mpg or 47.1 UK mpg.

Why is UK mpg higher than US mpg for the same car?

A UK imperial gallon is 4.546 liters and a US gallon is 3.785 liters. More fuel per gallon means more miles per gallon. Multiply US mpg by 1.20095 to get UK mpg.

Is 50 UK mpg good?

50 UK mpg is 41.6 US mpg, or 5.65 L/100km. That is the range of many small petrol cars and hybrids. Convert any figure before you compare cars rated in different countries.

Which countries use L/100km?

Most of Europe, Canada, Australia and China rate cars in L/100km. The US uses US mpg, and the UK still advertises UK mpg. Japan and India mostly use km/L.