Fuel Economy Converter

Background.

A British road test that praises a car for returning 50 mpg and an American window sticker reading about 41.6 mpg can be describing exactly the same fuel use. Neither is wrong. The UK quotes miles per Imperial gallon of 4.54609 litres, the United States quotes miles per US gallon of 3.785411784 litres, and the bigger British gallon carries a car about 20% further. Across most of Europe, Canada and Australia the same car would be listed at about 5.65 litres per 100 kilometres; in Japan or India it would more likely be quoted in kilometres per litre.

Those four units fall into two families that run in opposite directions. Miles per gallon and kilometres per litre measure distance per unit of fuel, so a bigger number is better. Litres per 100 km measures fuel per unit of distance, so a smaller number is better, and converting between the families means taking a reciprocal rather than multiplying by a factor. Enter a figure in any of the four units and the other three appear, each derived from the exact legal definitions of the mile and the two gallons.

The direction matters for more than labelling. Fuel bought tracks consumption, not economy, so equal steps in mpg are not equal savings. Moving from 20 to 30 US mpg saves 1.67 gallons every 100 miles; moving from 30 to 40 saves 0.83, and from 40 to 50 only 0.5, although each step looks the same on paper. The US Environmental Protection Agency prints gallons per 100 miles on its fuel economy label for that reason, and anyone deciding whether a more economical car is worth paying for will find the consumption figure the clearer guide.

What is fuel economy converter?

Fuel economy is distance travelled per unit of fuel; fuel consumption is its reciprocal, fuel used per unit of distance. US mpg counts statute miles per US liquid gallon, exactly 3.785411784 litres. Imperial mpg counts miles per Imperial gallon, fixed in UK law by the Weights and Measures Act 1985 at exactly 4.54609 litres. The two gallons parted company long ago: the American one descends from the old English wine gallon of 231 cubic inches, while Britain replaced its assortment of gallons in 1824 with the Imperial gallon, originally the volume of ten pounds of water. The mile itself is exactly 1.609344 kilometres in both countries. Kilometres per litre is the metric economy unit, and litres per 100 kilometres the metric consumption unit used on European, Canadian and Australian vehicle labels. Because every definition involved is exact, conversion between these units adds no uncertainty: whatever uncertainty there is lives in the original measurement. The US has a consumption unit of its own, gallons per 100 miles, which is simply 100 ÷ mpg and appears on EPA labels beside the mpg rating. All four units handled here describe liquid fuel by volume, so they say nothing about the energy in that fuel or the cost of buying it.

How to use this calculator.

1. 01Work out which unit your figure is in. An mpg figure from a UK source is almost always Imperial; one from a US source is US. Labels in Europe, Canada and Australia give L/100 km.
2. 02Enter the value and select its unit. Any positive number is accepted. A trip-computer reading converts like any other figure, but it is the car's own estimate rather than a measured fill-up.
3. 03Read the other three units. The US mpg figure is shown first, alongside Imperial mpg, L/100 km and km/L. One decimal place is plenty for comparing cars; the extra digits only matter when you convert a figure back.
4. 04To compare running costs, use L/100 km: multiplying it by the price of a litre gives the fuel cost of every 100 km, and the difference between two cars is a direct difference in fuel bought.
5. 05When combining several trips or tanks, add up total distance and total fuel rather than averaging the mpg figures, then convert the result.

The formula.

Every input is first turned into kilometres per litre, and the other units are calculated from that. From US mpg: km/L = mpg × 1.609344 ÷ 3.785411784, which works out to about 0.4251 km/L per mpg. From Imperial mpg the divisor is 4.54609 instead, about 0.3540 km/L per mpg. From L/100 km: km/L = 100 ÷ (L/100 km). Converting back uses the same constants the other way round, and L/100 km is always 100 divided by km/L.

Two shortcuts follow from those constants. Litres per 100 km equals 235.2146 divided by US mpg, or 282.4809 divided by Imperial mpg, so 30 US mpg is 235.2146 ÷ 30 = 7.8405 L/100 km. Between the two economy units the factors are fixed: 1 km/L equals 2.3521 US mpg or 2.8248 Imperial mpg. And Imperial mpg is US mpg multiplied by 4.54609 ÷ 3.785411784, about 1.2009, because the distance stays the same while the gallon grows by just over 20%.

The reciprocal is the part with consequences. Since L/100 km = 235.2146 ÷ US mpg, the fuel saved by a better figure shrinks as the figure rises: 20 mpg is 11.7607 L/100 km, 30 mpg is 7.8405 and 40 mpg is 5.8804. The first ten-mpg step saves about 3.92 litres every 100 km, the second only 1.96. In gallons per 100 miles, which is simply 100 ÷ mpg, 10 mpg uses 10 gallons, 20 uses 5, 30 uses 3.33 and 50 uses 2. Percentage claims shrink the same way: a car with 20% better mpg uses 1 − 1 ÷ 1.2 = 16.7% less fuel over the same distance, and a 10% improvement saves 9.1%.

Nothing here models how the original figure was measured. A test-cycle rating, a trip-computer reading and a tank-to-tank calculation convert identically, but they remain different measurements. Real consumption also moves with speed, cold starts, stop-start traffic, load, tyre pressure and air-conditioning, and none of that belongs in a change of units.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NIST SP 811 Appendix B.9: gives the mile-to-kilometer factor and U.S. MPG-to-km/L and L/100 km relationships.
2. [2]NIST Handbook 130 (2026), §2.6.1: gives 3.785411784 liters per U.S. gallon.
3. [3]UK Weights and Measures Act 1985, Schedule 1 Part IV: defines the Imperial gallon as 4.54609 cubic decimeters.
4. [4]U.S. EPA, Text Version of the Gasoline Label: explains MPG, consumption units and their inverse comparison behavior.
5. [5]Natural Resources Canada, 2013 Fuel Consumption Guide, Conversions: independently publishes MPG and L/100 km relationships and distinguishes gallon sizes.