Fuel Economy Converter: MPG to L per 100 km

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How to use this tool

Enter a figure and say what it is measured in. Everything else updates at once, including both kinds of mpg, litres per 100 km and kilometres per litre. The trip boxes at the bottom turn the economy figure into an actual quantity of fuel and an actual cost, which is usually the thing you wanted to know.

Miles per imperial gallon is a separate option rather than a footnote. British and Canadian figures quoted in mpg have historically used the imperial gallon, which is 20 percent larger, so a car advertised at 50 mpg in the United Kingdom is doing about 42 mpg by American measurement. Neither figure is wrong, they are just different units wearing the same abbreviation.

Why this conversion is a division

Most unit conversions multiply. This one does not, because the two units are inverses of each other. Miles per gallon measures distance for a fixed amount of fuel. Litres per 100 km measures fuel for a fixed distance. One goes up as the other goes down, so converting means dividing into a constant rather than multiplying by one.

The constant is 235.2145833 for US gallons. It comes straight from the definitions: a US gallon is 3.785411784 litres and a mile is 1.609344 km, both exact, so dividing one by the other and multiplying by 100 gives that figure with no rounding. For imperial gallons the constant is 282.4809363. Divide either one by your mpg and you have litres per 100 km, and dividing the other way gets you back.

The trap that costs people money

Because the scale is inverted, equal steps in mpg are not equal savings, and the difference is dramatic rather than subtle.

Over 10000 km, moving from 10 mpg to 20 mpg saves 1176 litres. Moving from 40 mpg to 50 mpg, the same ten mile per gallon improvement, saves 118 litres. That is a factor of ten. Replacing one old pickup with a merely average one saves more fuel than replacing ten efficient cars with very efficient ones, which is the opposite of most people's intuition and has real consequences for how you spend money on a fleet.

Litres per 100 km has no such problem. It measures fuel consumed directly, so a drop of two litres per 100 km is the same amount of fuel saved wherever you start. That is the main argument for the metric convention, and the reason the chart above is worth a minute of your time.

A worked check you can repeat on paper

Take 30 mpg US. Divide 235.2145833 by 30 and you get 7.840 litres per 100 km. Now go the long way as a check: 30 miles is 48.28 km, and one US gallon is 3.7854 litres, so the car uses 3.7854 litres to cover 48.28 km. Scale that to 100 km by multiplying by 100 and dividing by 48.28, and you get 7.840 litres. Same answer, arrived at without the constant.

Then check the imperial figure. 30 miles per imperial gallon uses 4.54609 litres over the same 48.28 km, which is 9.416 litres per 100 km. Dividing 282.4809 by 30 gives the same 9.416, as it should.

Mistakes that catch people out

Comparing US and imperial mpg directly. They differ by 20 percent. A 40 mpg imperial car is a 33.3 mpg US car. Any table that just says mpg without saying which gallon is incomplete.

Averaging mpg figures. You cannot take the mean of two mpg numbers and get the combined economy, because the scale is inverted. A trip half at 20 mpg and half at 40 mpg averages 26.7 mpg, not 30. Averaging litres per 100 km works correctly, which is another point in its favour.

Assuming the dashboard readout is measured. Most trip computers estimate consumption from injector timing rather than measuring fuel used, and they tend to read optimistic. Filling the tank and dividing distance by litres added is the honest method.

Ignoring which cycle a figure came from. City, highway and combined ratings are different tests, and the older US EPA figures before 2008 were measurably optimistic compared with the current ones. Converting a number does not make it comparable to a number from a different test.

Popular fuel economy conversions

The figures people look up most often. Each opens the converter ready to go with its own address.

Miles per US gallon to litres per 100 km

Miles per imperial gallon

Litres per 100 km to mpg

Kilometres per litre

How this was checked

Both constants were derived from the underlying exact definitions rather than copied. The US figure came out as 235.2145833 and the imperial as 282.4809363, matching the values published in conversion references. Round tripping was checked as well: converting an mpg figure to litres per 100 km and straight back returns the original number exactly, which it must for an inverse relationship implemented correctly.

The savings figures in the chart were computed rather than estimated. At 10 mpg a vehicle uses 2352 litres per 10000 km and at 20 mpg it uses 1176, a saving of 1176 litres. At 40 mpg it uses 588 and at 50 mpg 470, a saving of 118. The ratio between those two savings is very close to ten, which is the point the chart is making.

Related Calculators and Reference

• Volume and Capacity Converter for the gallons underneath these figures, and why there are two of them.
• Kmh to Mph Converter for the speed side of the same imperial and metric split.
• Length and Distance Converter for the mile and kilometre definitions this rests on.
• Unit Price Comparison Calculator for comparing fuel prices quoted per litre against prices quoted per gallon.
• Gas Cost Calculator: fuel cost of a trip, per person and per mile.
• EV Charging Time and Cost Calculator: compare mpg with electricity used per 100 km.
• Engine Compression Ratio and Displacement Calculator: the displacement and compression ratio behind an engine.
• Tire Size Calculator: correct odometer miles after a tire size change before working out fuel economy.
• NIST Special Publication 811 is the primary reference for the exact gallon and mile definitions used here.