Fuel Economy Converter
Convert fuel economy across the four units the world actually uses: MPG (UK imperial gallon), MPG (US gallon), litres per 100km, and km per litre. Edit any one field and the other three update live. Handy for reading European spec sheets in British numbers, comparing imported cars, or sanity-checking what a North American review really means.
Explain like I'm 5 (what even is this calculator?)
Different countries measure how thirsty a car is in different ways. The UK uses one kind of gallon, the US uses a smaller one, most of Europe counts litres per 100 kilometres instead of miles per gallon, and a few countries do kilometres per litre. Same car, four different numbers. Type any one of them and this page works out the other three.
Convert
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Prove it (show the working)
The calculator anchors every result on L/100km, then converts out. That keeps round-tripping stable and the maths auditable in one place.
Conversion factors
- 1 imperial gallon = 4.54609 litres (defined value)
- 1 US gallon = 3.785411784 litres (defined value)
- 1 mile = 1.609344 kilometres (defined value)
- MPG (UK) to L/100km factor: (4.54609 × 100) ÷ 1.609344 = 282.4809...
- MPG (US) to L/100km factor: (3.785411784 × 100) ÷ 1.609344 = 235.2146...
Formula chain
- MPG (UK) → L/100km: 282.481 ÷ mpg_uk
- MPG (US) → L/100km: 235.215 ÷ mpg_us
- km/L → L/100km: 100 ÷ kml
- L/100km → MPG (UK): 282.481 ÷ l100km
- L/100km → MPG (US): 235.215 ÷ l100km
- L/100km → km/L: 100 ÷ l100km
Worked example: 60 MPG (UK)
- L/100km = 282.481 ÷ 60 = 4.71 L/100km
- MPG (US) = 235.215 ÷ 4.7080 = 49.96 MPG (US)
- km/L = 100 ÷ 4.7080 = 21.24 km/L
Sense-check: a 60 MPG (UK) car should look thirstier in US MPG (it is, 50 MPG-US) because the US gallon is smaller, and should be in the low single digits in L/100km because the car is fairly efficient.
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Why MPG (UK) and MPG (US) are not the same number
Both numbers are miles per gallon. The catch is that the gallon is not the same volume on either side of the Atlantic. The UK kept the imperial gallon, defined as exactly 4.54609 litres. The US uses a gallon based on the older Queen Anne wine gallon, defined as exactly 3.785411784 litres. The US gallon is roughly 17 per cent smaller. So the same car burning the same fuel over the same distance scores about 17 per cent fewer MPG in US units than in UK units. A British car magazine quoting 50 MPG and an American magazine quoting 50 MPG are not describing the same car. Translate before you compare.
Why L/100km is "inverted" relative to MPG
MPG asks "how far does a fixed amount of fuel take you?" Bigger numbers mean a more efficient car. L/100km asks the opposite question: "how much fuel does it take to cover a fixed distance?" Smaller numbers mean a more efficient car. The two units are reciprocal, but the framing matters in practice. Drops in L/100km at the thirsty end of the scale represent more fuel saved per year than drops at the efficient end. Going from 12 L/100km to 10 L/100km saves you a lot more diesel over 15,000 km than going from 5 L/100km to 4 L/100km, even though the second drop looks like a bigger proportional improvement. MPG hides that effect because it compresses thirsty cars into a narrow band at the bottom of the scale.
Who uses which standard
The UK is the main holdout for MPG with the imperial gallon. New car spec sheets, the DVLA, motoring journalists and most consumer reviews still use it. The US and Canada use MPG with the US gallon, although Canadian official window stickers also publish L/100km, which is gradually winning. Most of the European Union, plus Australia and New Zealand, use L/100km exclusively. India, Japan and several South American countries lean on km/L. If you are buying a car shipped in from another market, or reading a specification sheet from a manufacturer's home market, the published economy figure is rarely in the unit you grew up with.
Common gotchas when comparing imported cars
The first trap is assuming MPG is MPG. A 40 MPG figure on a US car becomes about 48 MPG on the UK scale. A 40 MPG figure on a UK car drops to about 33 MPG on the US scale. The second trap is mixing test cycles. UK and EU figures historically used NEDC, then WLTP, both lab cycles that flatter modern cars. US EPA figures come from a different cycle that tends to read lower. Two cars with the same real-world economy can advertise wildly different headline numbers because of the cycle, before any unit conversion is applied. Use this converter to put the published figures on the same scale, then knock 10 to 20 per cent off WLTP figures to land somewhere closer to a real motorway average.
Edge cases this calculator handles
Zero or negative input blanks the other three fields rather than dividing by zero. A very high MPG figure (a hypermiler claiming 100 MPG-UK) produces a believably small L/100km, around 2.82. A very low MPG figure (a thirsty pickup at 5 MPG-US) produces a believably large L/100km, around 47. Round-tripping is stable: typing a value into one field, copying the resulting value out of another, and pasting it back in any other field will return the original number to within rounding. The internal calculation is anchored on L/100km to avoid drift between the two MPG units.
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Frequently asked questions
Why is MPG (UK) different from MPG (US)?
The two countries use different gallons. The imperial gallon is 4.54609 litres, the US gallon is 3.785411784 litres, so a US gallon is about 17 per cent smaller. A car that does 30 MPG (UK) does roughly 25 MPG (US) for the same fuel use, because the US is dividing the same miles by a smaller gallon. Always check which gallon a published figure is using before comparing two cars.
Why is L/100km the opposite way around to MPG?
MPG measures distance per fixed fuel: bigger is better. L/100km measures fuel per fixed distance: smaller is better. The two are reciprocal, which is why a small drop in L/100km at the efficient end is worth less than the same drop at the thirsty end. Going from 10 to 9 L/100km saves more fuel over a year than going from 5 to 4 L/100km, even though both look like a one-unit improvement.
Which countries use which unit?
The UK uses MPG with the imperial gallon. The US and Canada use MPG with the US gallon, although Canada also publishes L/100km on official window stickers. Most of mainland Europe and Australia use L/100km. India, Japan and a few South American countries lean on km/L. If you are comparing a car spec sheet against a number you grew up with, you almost certainly need a conversion.
Are the constants on this page exact?
The underlying volume and distance constants are exact by definition: 1 imperial gallon is 4.54609 litres, 1 US gallon is 3.785411784 litres, and 1 mile is 1.609344 kilometres. The combined MPG-to-L/100km factors (282.481 for UK, 235.215 for US) are rounded for display, but the calculator uses the full-precision derivations internally and only rounds at the end. The Conversion factors block above lists the sources so you can audit the working.
Why does MPG round to one decimal but L/100km to two?
MPG figures published by manufacturers and motoring journalists almost always quote one decimal place, sometimes none. L/100km figures are routinely quoted to two decimals because the unit is smaller in absolute terms and a one-decimal version loses resolution at the efficient end (a 4.2 versus 4.3 L/100km difference is real). The rounding mirrors the convention each market actually uses.