Fuel Economy Converter

Enter a fuel economy figure, choose the unit it is quoted in, and the calculator restates it in the four measures drivers meet: miles per US gallon, miles per imperial gallon, litres per 100 kilometres and kilometres per litre. The headline result is litres per 100 km; the table beneath carries the full set. MPG and km/L measure distance per amount of fuel, so bigger is better; L/100 km measures fuel per distance, so smaller is better.

How the conversion works

Every input is first expressed as litres per 100 kilometres, and the other three are read back off that value. For a figure in miles per US gallon the bridge is:

L/100 km = 235.214583 ÷ MPG_US

The constant is not a rounded convenience. A US gallon is exactly 3.785411784 litres and a mile is exactly 1.609344 kilometres, so a car doing M miles per US gallon burns 3.785411784 litres every 1.609344 × M kilometres. Scaling that to 100 kilometres gives 100 × 3.785411784 ÷ 1.609344 = 235.214583 litres, divided by M; NIST's guide to the SI publishes the same divisor rounded to 235.215. The imperial constant is built the same way from the 4.54609-litre imperial gallon: 100 × 4.54609 ÷ 1.609344 = 282.480936. Kilometres per litre is the same reciprocal scaled by 100, so L/100 km equals 100 divided by km/L, and the gallon ratio 4.54609 ÷ 3.785411784 = 1.20095 converts US MPG to imperial MPG directly.

The default input runs the whole chain. A car rated at 30 MPG US consumes 235.214583 ÷ 30 = 7.84 L/100 km. Its imperial rating is 30 × 1.20095 = 36.03 MPG, a fifth higher for the identical car, purely because the imperial gallon holds a fifth more fuel. In kilometres per litre it manages 100 ÷ 7.840486 = 12.75 km/L, and a second route confirms it: 30 miles is 48.28032 km, and 48.28032 ÷ 3.785411784 lands on the same 12.75. The identity runs both ways, since a US MPG figure multiplied by its L/100 km equivalent always returns 235.214583, which is why halving one measure doubles the other.

Two gallons and one mile

The awkward part is not the mile, which every MPG country shares, but the gallon, which splits in two. The American gallon is the old English wine gallon of exactly 231 cubic inches, in customary use for centuries and fixed by a statute of Queen Anne passed in 1706 and in force from 1707. The odd figure probably began as workshop geometry: with pi taken as 22/7, a cylinder 7 inches across and 6 inches deep holds exactly 231 cubic inches. With the modern 2.54-centimetre inch, 231 cubic inches is exactly 3,785.411784 cubic centimetres, the source of every decimal place in the US gallon constant. Britain's Weights and Measures Act of 1824 scrapped the wine gallon and defined a single imperial gallon as the volume of 10 pounds of distilled water, weighed in air against brass weights with the barometer at 30 inches of mercury and the water at 62 degrees Fahrenheit, a quantity the Weights and Measures Act 1985 fixed at exactly 4.54609 litres. The United States, independent since 1776, stayed with the wine gallon, which the Treasury adopted as its standard of liquid measure in 1832 and a congressional resolution of June 1836 sent to every state governor.

The consequence is a permanent 20 percent gap between two units that share a name. A car achieving 40 imperial MPG in a British review achieves 33.31 US MPG, with not a drop of fuel differing between them. The mile, at least, is settled: since the International Yard and Pound Agreement signed on 1 July 1959 it has been exactly 1.609344 kilometres everywhere MPG is used, a story told in more detail on the speed converter page.

The oil crisis and the lower-is-better school

MPG owes its regulatory weight in the United States to a specific shock. OPEC's Arab members embargoed oil shipments to the United States from October 1973 to March 1974, and the price of oil quadrupled. Congress answered with the Energy Policy and Conservation Act, signed by President Gerald Ford on December 22, 1975, which created the Corporate Average Fuel Economy program, known as CAFE: fleet-average targets for carmakers, denominated in miles per US gallon and intended to roughly double the new-car average to 27.5 MPG by model year 1985. The standards did not begin with the signing: the Union of Concerned Scientists' history of US fuel efficiency rules dates the first passenger-car requirement to model year 1978, and the National Highway Traffic Safety Administration, which administered it, set that opening figure at 18.0 MPG before raising it year by year. From then on MPG carried legal force in the United States, and higher meant compliant.

Europe standardised on the opposite convention: litres per 100 kilometres, where lower is better, and EU law has required the figure on new-car labels since Directive 1999/94/EC of 13 December 1999. The volume-per-distance form answers the question a driver budgets for, how much fuel a trip will take, and it adds linearly: a 400 km drive in an 8 L/100 km car takes 32 litres, twice the 200 km figure. MPG answers the complementary question of how far a tank will stretch. Neither is wrong, but intuition trained on one behaves badly on the other.

The MPG illusion

That misbehaving intuition has a name. Writing in Science on 20 June 2008, in volume 320 at pages 1593 to 1594, Richard Larrick and Jack Soll showed that people judge fuel savings as if they scaled linearly with MPG, when fuel used is actually the reciprocal of MPG. Their finding is about how savings are perceived, not a claim that MPG measures the wrong thing. The arithmetic makes the trap concrete: over 10,000 miles, a pickup improved from 12 to 15 US MPG goes from about 833 gallons to about 667, saving 167; a sedan improved from 30 to 40 goes from 333 gallons to 250, saving 83. The unglamorous 3-MPG gain saves twice the fuel of the 10-MPG one, and intuition tends to rank them the other way round. The EPA's fueleconomy.gov label guidance illustrates the same effect at smaller scale: improving from 10 to 15 MPG saves about 33 gallons every 1,000 miles, while improving from 30 to 35 saves about 5.

Express the same vehicles in this calculator's primary unit and the illusion dissolves. 12 US MPG is 19.60 L/100 km and 15 is 15.68, a drop of 3.92 litres per 100 km; 30 is 7.84 and 40 is 5.88, a drop of 1.96, exactly half as much and visibly so. On the consumption scale the thirsty-end upgrade is plainly the one that matters. The label design partly followed suit: the window sticker the EPA and NHTSA redesigned for model year 2013 states fuel consumption per 100 miles alongside MPG.

What a conversion cannot tell you

The four measures here are one physical quantity in different clothes, and translating between them is exact. What is not exact, or even meaningful, is treating a converted number as a rating under a different testing regime. EPA figures in the United States, WLTP figures in Europe and the NEDC figures WLTP replaced for new EU type approvals from September 2017 each come from their own laboratory driving cycles, and the same car earns genuinely different numbers under each. Converting a WLTP consumption figure into MPG produces that same WLTP measurement in American units, not the number an EPA test would have printed. Ratings are not cross-convertible; only units are.

The other habitual trap is the unlabelled gallon. A British road test and an American spec sheet can quote two MPG figures for one engine, and the 1.20095 gallon ratio is large enough to make a mediocre car look excellent. British pumps sell fuel by the litre, yet the UK's official new-car labels still print imperial MPG alongside L/100 km, so the imperial figure stays in circulation. When a claimed figure seems too good, its L/100 km equivalent usually settles which gallon was meant. To turn a consumption figure into money for a specific journey, the fuel cost calculator picks up where this page stops.

Assumptions and conventions

All three underlying constants are exact by definition, not measured: 3.785411784 litres per US gallon, 4.54609 litres per imperial gallon and 1.609344 kilometres per mile. The derived constants 235.214583 and 282.480936 follow from them arithmetically, and both are repeating decimals shown here rounded to six places, while the calculator computes at full floating-point precision and rounds only the displayed values to two decimals. The input accepts values from 0.1 up to 1,000 in any of the four units; zero and negative values are rejected, since none of the measures can represent them, and a non-numeric entry produces no result. The table never highlights a best row: the four lines are one measurement restated, not competing alternatives.

Frequently asked questions

What is 30 MPG in litres per 100 km?

30 US MPG is 7.84 L/100 km, because L/100 km equals 235.214583 divided by the US MPG figure. 30 imperial MPG is 9.42 L/100 km, using the imperial constant of 282.480936. The two answers differ because an imperial gallon holds 4.54609 litres against 3.785411784 litres for the US gallon.

How do I convert MPG to L/100 km?

Divide 235.215 by a US MPG figure, or 282.481 by an imperial one; 235.215 is the rounded divisor NIST itself publishes in its SP 811 conversion tables. The constants come from exact definitions, 100 times the litres in the gallon divided by 1.609344 kilometres per mile, so the conversion itself introduces no error. Going the other way is the same division, since the two measures are reciprocals of each other.

Why is a US gallon smaller than an imperial gallon?

The United States kept the old English wine gallon of 231 cubic inches, which works out to 3.785411784 litres, while Britain replaced its old gallons in 1824 with the larger imperial gallon, which the Weights and Measures Act 1985 fixes at exactly 4.54609 litres. The ratio is 1.20095, so the same car scores about 20 percent more MPG in a UK brochure than in a US one.

What is the MPG illusion?

It is the finding, published by Richard Larrick and Jack Soll in Science on 20 June 2008, that people wrongly judge fuel savings as if they rose linearly with MPG. Fuel used is the reciprocal of MPG, so over 10,000 miles a pickup improved from 12 to 15 US MPG saves about 167 gallons, while a sedan improved from 30 to 40 saves only about 83, half as much for a gain more than three times as large in MPG.

Can I convert an EPA rating into a WLTP or NEDC figure?

No. This converter changes units exactly, but EPA, WLTP and NEDC ratings come from different test procedures, so the same car earns genuinely different numbers under each. Converting a WLTP result into MPG gives you that same WLTP measurement in American units, not the figure an EPA test would have produced.

Is a higher or lower fuel economy number better?

It depends on the unit. MPG and km/L measure distance per amount of fuel, so higher is better. L/100 km measures fuel per distance, the convention used across Europe, so lower is better. A car improving from 8 to 6 L/100 km is getting more efficient even though the number is falling.