The conversion
Miles per gallon and litres per 100 km measure the same thing from opposite ends: one counts distance per unit of fuel, the other counts fuel per unit of distance. Because the imperial gallon is 4.54609 litres and a mile is 1.609344 km, the two are linked by a single magic number:
The same division works in both directions, which makes it easy to remember. It also means the scales run opposite ways: a higher mpg is better, but a lower litres per 100 km is better. That trips people up when comparing car brochures, where official figures are usually quoted per 100 km.
Britain manages to use both at once: fuel is sold by the litre, distances are signed in miles, and yet everyone still talks about miles per gallon even though nobody has bought a gallon of petrol in decades. The tool also shows two tidier hybrids of the systems, kilometres per litre and miles per litre. Miles per litre is arguably the most practical figure of all for a UK driver, because it multiplies directly against the pump price: at 8.8 miles per litre and £1.45 a litre, each mile costs about 16.5 pence.
Worked examples
Quick reference
- 30 mpg ≈ 9.42 L/100km
- 35 mpg ≈ 8.07 L/100km
- 40 mpg ≈ 7.06 L/100km
- 45 mpg ≈ 6.28 L/100km
- 50 mpg ≈ 5.65 L/100km
- 55 mpg ≈ 5.14 L/100km
- 60 mpg ≈ 4.71 L/100km
- 70 mpg ≈ 4.04 L/100km
Mind the American gallon
US mpg figures are not comparable with UK ones. The US gallon is about 3.785 litres against the imperial gallon's 4.54609 litres, so the identical car scores roughly 20 percent lower in American tests. Multiply a US figure by 1.201 to get the UK equivalent: a car rated 33.3 mpg in the US is doing 40 mpg in UK terms. Whenever you read an American car review, remember their 30 mpg is our 36.
Fuel economy is also where small differences quietly become real money. Driving 8,000 miles a year at 45 mpg uses about 808 litres of petrol, roughly £1,170 at £1.45 a litre; the same mileage at 35 mpg needs about 1,039 litres and costs about £335 more. Real-world consumption normally sits below the official WLTP figure, so treat brochure numbers as a best case and compare cars on the same basis. Driving style moves the figure too: gentle acceleration, anticipating traffic and keeping tyres properly inflated can be worth several mpg, which is by far the cheapest fuel saving available to any driver. For the distance side of the sum, our miles to km converter has the running and driving references.
Frequently asked questions
How do I convert mpg to litres per 100 km?
Divide 282.481 by the mpg figure. Handily the same formula works in reverse: divide 282.481 by the litres per 100 km to get mpg back. So 40 mpg is 7.06 L/100km, and 7.06 L/100km is 40 mpg.
Is UK mpg the same as US mpg?
No. The imperial gallon is 4.54609 litres but the US gallon is only about 3.785 litres, so the same car scores about 20 percent higher in UK mpg. Multiply a US figure by 1.201 to get the UK equivalent, or divide a UK figure by 1.201 for the US one.
What is a good mpg for a car?
As a rough guide in UK mpg, over 50 is very economical for a petrol car, 40 to 50 is typical, and under 30 is thirsty. Hybrids often exceed 60 mpg in mixed driving. Real-world figures are usually below the official test numbers.
Why do car adverts quote litres per 100 km?
Official European fuel economy tests report consumption as litres per 100 km, and lower is better, whereas with mpg higher is better. UK brochures often show both. This tool lets you type either figure and read off the other instantly.
How much does fuel economy affect cost?
Directly. At 40 mpg, 100 miles uses 2.5 imperial gallons, about 11.4 litres, which costs around £16.50 at £1.45 a litre. The same trip at 50 mpg uses a fifth less fuel, so small mpg differences add up over a year of driving.