This fuel economy converter turns one figure into the other two units a driver in the United States, the United Kingdom, Canada, or Australia is likely to meet. Enter a number and say whether it is US miles per gallon, UK miles per gallon, or litres per 100 kilometres. The sample is 30 US mpg. The headline is 7.84 L/100 km. The same car is 36.03 miles per imperial gallon. The length converter on this site multiplies metres by a foot factor. It does not know about gallons. This page does not convert a distance. It converts a rate of fuel use.
The trap is that “miles per gallon” names two gallons. A US gallon is exactly 231 cubic inches. With the inch defined as 0.0254 metres, that gallon is 3.785411784 litres. An imperial gallon, the one UK fuel economy still uses, is exactly 4.54609 litres. The imperial gallon is larger, so the same car posts a higher mpg number in Britain than it does in the United States. Thirty US mpg and 30 UK mpg are not the same consumption. Thirty UK mpg is 9.42 L/100 km. Canada and Australia publish litres per 100 km, where a larger number is thirstier. The United States publishes mpg, where a larger number is more frugal. Reading either scale backwards is how a “good” figure becomes a bad one.
How to use this calculator
- Enter the figure you already have. The sample, 30, is a fuel-economy number, not a distance and not a price per gallon.
- Choose the unit that figure is in: US miles per gallon, UK miles per gallon, or litres per 100 km. The sample is the US gallon.
- Press Calculate. If you started from either mpg, the headline is litres per 100 km. If you started from litres per 100 km, the headline is US mpg.
- Read all three lines under the headline before you compare a US window sticker with a UK brochure or a Canadian EnerGuide label. The line after the starting figure shows what the same digits would mean in the other gallon.
How mpg becomes litres per 100 km
Litres per 100 km equals the litres in the gallon, times 100, divided by mpg times the kilometres in a mile. The mile is the international mile, 1.609344 kilometres, the same mile the 5K pace page uses. For a US gallon that quotient at 1 mpg is about 235.214583. The calculator does not round the constant first. Thirty US mpg divides out to 7.840457 before rounding, and the display shows 7.84. The UK figure uses 4.54609 litres instead of 3.785411784, so the 1 mpg constant is larger, about 282.480936. Thirty UK mpg divides out to a figure the display shows as 9.42.
Going the other way divides the same constant by the L/100 km figure. The sample’s 7.84 L/100 km converts back to 30.00 US mpg and 36.03 UK mpg. That return trip works because 7.84 was already the rounded US result. A figure you measured independently, such as 8.00 L/100 km, is 29.40 US mpg and 35.31 UK mpg. It will not land on a whole number of miles per gallon, and the calculator does not force it to. Six and a half litres per 100 km, a figure you might see on an Australian label, is 36.19 US mpg and 43.46 UK mpg. The display writes the 6.5 you typed as 6.50, so the unit line is unambiguous.
The same digits in the wrong gallon are a different car. Thirty US mpg is 7.84 L/100 km. Thirty UK mpg is 9.42 L/100 km. The gap is 1.58 litres per 100 km, which is the whole difference between the two gallons: the imperial gallon holds 4.54609 / 3.785411784, or about 1.201, times as much fuel. A UK brochure that says 40 mpg is 7.06 L/100 km and 33.31 US mpg. A US window sticker that says 40 mpg is 5.88 L/100 km and 48.04 UK mpg. Treating the US 40 as if it were imperial makes the car look thirstier than it is, at 7.06 L/100 km instead of 5.88. Treating the UK 40 as if it were American makes the car look more frugal than it is.
Fifty US mpg, a common hybrid claim, is 4.70 L/100 km and 60.05 UK mpg. The same digits read as UK mpg would be 5.65 L/100 km, not 4.70. None of these conversions includes the price of fuel, a tank size, or a trip distance. A car that uses 7.84 litres per 100 km uses 7.84 litres to travel 100 km, which is 62.14 miles. The page does not multiply that by a fuel price. The length converter will turn a trip distance from metres into miles if you need the distance first. It will not turn the mpg into a fuel cost either.
Formula
1 US gallon = 231 cubic inches = 3.785411784 litres 1 imperial gallon = 4.54609 litres 1 mile = 1.609344 kilometres L/100 km from US mpg = 3.785411784 × 100 / (mpg × 1.609344) L/100 km from UK mpg = 4.54609 × 100 / (mpg × 1.609344) US mpg from L/100 km = 3.785411784 × 100 / (L/100 km × 1.609344) UK mpg from L/100 km = 4.54609 × 100 / (L/100 km × 1.609344) UK mpg = US mpg × 4.54609 / 3.785411784 The headline and the detail lines show two decimal places.
The US-gallon constant often appears in print as 235.215. This page does not round the constant first. It divides the exact litre and mile definitions and then rounds the result to two decimal places. Thirty US mpg is 7.840457 before that rounding, so the headline 7.84 is the nearest hundredth, not a truncated 7.84 from 7.849. Fifty US mpg is 4.704292, which displays as 4.70.
Reversing a rounded headline can miss the original by a hundredth. Start from 7.84 L/100 km and the US result is 30.00 mpg, because 7.84 was this tool’s own rounding of 30. Start from a published 7.8 L/100 km, which has already been rounded by someone else, and the US mpg will not be exactly 30. The detail line “started from” shows the figure after it has been formatted, so 6.5 litres is shown as 6.50 L/100 km. The arithmetic uses the number you typed, not a second rounding of that line.
Worked fuel-economy conversions
30 miles per US gallon
Choose US miles per gallon and enter 30. The headline is 7.84 L/100 km. The detail lines are 30.00 US mpg, 36.03 UK mpg, and 7.84 litres per 100 km. The warning line says the same digits read as UK mpg would be 9.42 L/100 km. That 9.42 figure is a different car. It is what 30 imperial mpg consumes. It is not a second estimate of the US car.
Enter 7.84 and choose litres per 100 km. The headline flips to 30.00 US mpg, and the UK line is 36.03 again. The two directions agree at this hundredth. They agree because the converter rounded 30 US mpg to 7.84 and 7.84 converts back inside the same hundredth. A sticker that prints 7.8, or 7.840, can land a hundredth away. Compare the unit before you compare the digits.
The same “40 mpg” in each country
Forty US mpg shows 5.88 L/100 km and 48.04 UK mpg. The warning line says those same digits read as UK mpg would be 7.06 L/100 km. Run that case directly: enter 40 and choose UK miles per gallon. The headline is 7.06 L/100 km, the US line is 33.31, and the UK line stays 40.00. One brochure number has become two cars, 1.18 litres per 100 km apart.
The imperial gallon is the reason the UK mpg number is the larger one for a given car. Forty US mpg is 48.04 UK mpg, not 40 and not 33.31. The smaller US gallon makes the American mpg number look worse. A driver moving from Houston to Manchester who keeps the US number and calls it “mpg” on a UK forum has understated the imperial figure. A driver moving the other way who keeps the UK number has overstated the US figure. The three lines exist so that handoff does not depend on memory.
An Australian 6.5 L/100 km, and a 50 mpg hybrid claim
Enter 6.5 and choose litres per 100 km. The headline is 36.19 US mpg. The UK line is 43.46, and the litre line displays 6.50. The warning line says 6.50 L/100 km is 36.19 on a US gallon and 43.46 on an imperial gallon, and that those mpg figures are not interchangeable. Eight litres per 100 km, a rounder comparison figure, is 29.40 US mpg and 35.31 UK mpg. Both litre figures are thirstier as the number rises. Both mpg figures are thirstier as the number falls. The converter does not invert that relationship for you beyond showing all three.
Enter 50 and choose US miles per gallon. The headline is 4.70 L/100 km, and the UK line is 60.05. The same digits read as UK mpg would be 5.65 L/100 km. A hybrid claim printed as “50 mpg” in a US advertisement is the 4.70 figure. The same claim printed in a UK advertisement would be the 5.65 figure, which is a thirstier car. The page does not know which advertisement you saw. The unit menu is that decision. It also does not know whether the test cycle was a laboratory figure or a trip you measured. The arithmetic is the same either way. The real-world gap between a test cycle and a commute is outside the gallon definition.
The same fuel use in three units
| Start | US mpg | UK mpg | L/100 km |
|---|---|---|---|
| 30 US mpg | 30.00 | 36.03 | 7.84 |
| 30 UK mpg | 24.98 | 30.00 | 9.42 |
| 40 US mpg | 40.00 | 48.04 | 5.88 |
| 40 UK mpg | 33.31 | 40.00 | 7.06 |
| 50 US mpg | 50.00 | 60.05 | 4.70 |
| 6.50 L/100 km | 36.19 | 43.46 | 6.50 |
| 8.00 L/100 km | 29.40 | 35.31 | 8.00 |
What this fuel-economy conversion leaves out
The page converts a rate. It does not compute the fuel for a trip, the cost of a tank, or the range from a given tank size. To get litres for a journey you still multiply L/100 km by the kilometres and divide by 100. To get gallons you divide those litres by the gallon you buy, which is the US gallon in the United States and the imperial gallon in the United Kingdom. Canadian and Australian pumps sell litres, so the L/100 km line is already in the unit on the pump. Mixing a US mpg with a price per imperial gallon, or a UK mpg with a price per US gallon, puts the wrong gallon on both sides of the cost.
Laboratory test cycles differ by country and by year. A US window-sticker mpg, a WLTP figure converted into UK mpg, and an Australian L/100 km from a different cycle are not the same experiment even after the units match. This converter makes the units comparable. It does not make the test cycles comparable. A figure you computed from your own trip, distance divided by fuel added, is the figure the arithmetic expects. Whether that trip included a roof box, a cold start, or a tailwind is not in the form.
Older UK publications sometimes used miles per imperial gallon while a reader in the United States assumed the smaller gallon. Older Canadian publications sometimes used miles per imperial gallon as well, before litres per 100 km became the usual label. If a source does not say which gallon, the two results on this page are the range of that ambiguity, not a single answer. The length converter remains the tool for metres, feet, and kilometres of distance. A pace in minutes per mile is the 5K page. Neither of those pages divides by a gallon.
Very small inputs are still divided. A typo of 3 instead of 30 US mpg produces a huge L/100 km and is mathematically valid. The form rejects zero and negative numbers because a rate of fuel use has to be above zero. It does not reject a number that is merely implausible for a passenger car. Electric vehicles quoted in miles per kilowatt-hour or watt-hours per kilometre are a different rate. There is no gallon in that conversion, and this page will not invent one.
Frequently asked questions
What is 30 US mpg in litres per 100 km and in UK mpg?
30 US mpg is 7.84 L/100 km and 36.03 UK mpg. The US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres, so the same car earns a higher mpg number in the UK. The same digits read as UK mpg would be 9.42 L/100 km, which is a thirstier rate. Enter 7.84 L/100 km and the converter returns 30.00 US mpg.
Why is 40 US mpg not the same car as 40 UK mpg?
The imperial gallon is about 1.201 times the US gallon. Forty US mpg is 5.88 L/100 km and 48.04 UK mpg. Forty UK mpg is 7.06 L/100 km and 33.31 US mpg. The gap between those two litre figures is 1.18 L/100 km. A larger mpg number means a more frugal car only when both numbers use the same gallon.
How do I convert litres per 100 km to US miles per gallon?
Divide the exact US constant, 3.785411784 × 100 / 1.609344, by the L/100 km figure. Printed tables often round that constant to 235.215 first. This page divides the litre and mile definitions directly, then rounds the answer. Eight litres per 100 km is 29.40 US mpg and 35.31 UK mpg. Six and a half litres per 100 km is 36.19 US mpg and 43.46 UK mpg. A larger L/100 km figure is a thirstier car, the opposite direction from mpg.
Does 50 US mpg convert to 50 UK mpg?
No. Fifty US mpg is 4.70 L/100 km and 60.05 UK mpg. Reading those same digits as UK mpg would mean 5.65 L/100 km, not 4.70. A hybrid claim has to say which gallon before the number can be compared with a Canadian or Australian litres-per-100 km label. This page does not adjust for the difference between a laboratory cycle and a commute.
Can this fuel converter price a trip or convert metres to miles?
No. It converts a rate of fuel use among US mpg, UK mpg, and L/100 km. Multiplying by a distance and a price per litre is a separate step, and the price has to use the same gallon or the same litre you converted to. Distance in metres belongs on the length converter. A running pace belongs on the 5K pace page. Neither of those tools divides by a gallon.
Related ClearCalc pages
- Length converter from meters Metres to feet, inches, yards, or kilometres. It does not convert mpg or litres per 100 km.
- 5K pace calculator Minutes per kilometre and per mile from a finish time. A pace is not a fuel-economy rate. The mile is the same 1.609344 kilometre mile.
- Percentage calculator Useful if you want the gap between two L/100 km figures as a percent after you have converted them here.