Worldwide
Fuel economy converter
Fuel economy is quoted in different units around the world, and the figures look nothing like each other even when they describe the same car.
Fuel economy is quoted two opposite ways.
This converter moves between the four common ones: litres per 100 km used across most of Europe, kilometres per litre used in much of Asia and Latin America, miles per gallon in the UK using the imperial gallon, and miles per gallon in the United States using the smaller US gallon. Enter a figure in any one of them and it returns the equivalent in all four at once. The reason this matters is that the units run in opposite directions. Litres per 100 km measures how much fuel you burn to cover a fixed distance, so a lower number is better. Miles per gallon and kilometres per litre measure how far you travel on a fixed amount of fuel, so a higher number is better. Mixing them up makes an efficient car look thirsty or the reverse. The other classic trap is the gallon itself. The UK imperial gallon is 4.546 litres, while the US gallon is only 3.785 litres, about a fifth smaller. That means the very same car is quoted at a higher mpg in the UK than in the US, purely because the UK gallon holds more fuel. A car rated at 40 US mpg is about 48 UK mpg, with no change to the car at all. The conversions here use the exact legal definitions of the units, so the numbers line up precisely rather than by a rough rule of thumb.
How it works
- Type in your car economy figure.
- Choose which unit you entered it in: L/100km, km/L, UK mpg or US mpg.
- The tool converts your figure to litres per 100 km behind the scenes as a common base.
- It then expresses that same economy in all four units.
- Read across the tiles; the unit you typed is highlighted so you can spot it.
L/100km is the base; US mpg x (L/100km) = 235.214583, UK mpg x (L/100km) = 282.480936, km/L = 100 / (L/100km)
Every figure is first turned into litres per 100 km. Kilometres per litre is the reciprocal scaled to 100 km, so it equals 100 divided by the litres per 100 km. Miles per gallon is distance per unit of fuel, the reciprocal of fuel per distance, so multiplying a mpg figure by the matching constant returns litres per 100 km and dividing the constant by litres per 100 km returns mpg. The constants come from the exact mile and gallon definitions: 235.214583 for the US gallon and 282.480936 for the imperial gallon.
- L/100km
- litres of fuel to travel 100 km (lower is better)
- km/L
- kilometres travelled per litre (higher is better)
- UK mpg
- miles per imperial gallon of 4.54609 litres
- US mpg
- miles per US gallon of 3.785411784 litres
The same economy in every unit
| 5 L/100km | 20 km/L | about 56 UK mpg, 47 US mpg, an efficient car |
| 7 L/100km | 14.3 km/L | about 40 UK mpg, 34 US mpg, a typical car |
| 9 L/100km | 11.1 km/L | about 31 UK mpg, 26 US mpg, a thirstier car |
| 12 L/100km | 8.3 km/L | about 24 UK mpg, 20 US mpg, a large or older car |
Worked example
Your car uses 8 litres per 100 km and you want the other units: that is 12.5 km per litre, about 35.3 mpg on the UK imperial gallon, and about 29.4 mpg on the US gallon. The two mpg numbers differ by roughly a fifth even though the car is identical, because the imperial gallon is larger. Going the other way, a US figure of 30 mpg works out at about 7.84 litres per 100 km, and a UK figure of 40 mpg at about 7.06 litres per 100 km.
Key facts
- The UK imperial gallon is about 20 percent larger than the US gallon, so UK mpg reads higher for the same car.
- Litres per 100 km counts fuel for a set distance, so a lower figure is the better one.
- Miles per gallon and km per litre count distance per fuel, so a higher figure is better.
- The conversions use exact unit definitions: 1 mile is 1.609344 km and the US gallon is 3.785411784 litres.
Tips
- Always check whether an mpg figure is UK or US before comparing two cars; the gap is about a fifth.
- To estimate L/100km from mpg in your head, divide 282 by UK mpg or 235 by US mpg.
- Compare cars in one unit only, so you are not misled by the gallon difference.
- Treat maker figures as a best case; real economy is usually lower than the official test number.
Frequently asked questions
Why are UK and US mpg different?+
Because the gallon is not the same size. The imperial gallon used in the UK is 4.54609 litres, while the US gallon is 3.785411784 litres. Miles per gallon divides a fixed distance by the fuel in one gallon, so the larger UK gallon produces a higher mpg for the same real efficiency. UK mpg figures are about 20 percent higher than US ones for the same car.
Is a higher or lower number better?+
It depends on the unit. For miles per gallon and kilometres per litre, higher is better because you travel further on the same fuel. For litres per 100 km, lower is better because you burn less fuel over the same distance. The converter shows all four so you can read it whichever way you are used to.
How do I convert mpg to L/100km in my head?+
There is no simple multiply, because the two are reciprocals. As a rough guide, divide 282 by a UK mpg figure, or 235 by a US mpg figure, to get litres per 100 km. So 47 UK mpg is about 6 litres per 100 km. The tool does the exact sum for you.
Does this work for electric cars?+
Not directly. Electric efficiency is measured in energy, such as kilowatt-hours per 100 km or miles per kWh, not in litres or gallons of liquid fuel. This converter is for petrol, diesel and other liquid-fuel cars. An equivalent figure for an electric car would need an energy-based conversion instead.
Are these conversions exact?+
Yes. They use the defined values of the units: one mile is exactly 1.609344 km, the US gallon is exactly 3.785411784 litres, and the imperial gallon is exactly 4.54609 litres. The only rounding is in the final displayed figures, kept to two decimal places for readability.
Why does my car maker quote a different number?+
Official economy comes from standard test cycles, such as WLTP in Europe, run in controlled conditions. Real driving, with traffic, weather, load and driving style, usually returns less. The converter changes units faithfully, but it cannot turn a lab figure into your real-world economy.
Things to watch
- This converts units only; it does not estimate real-world economy, which depends on how and where you drive.
- It is for liquid-fuel cars; electric efficiency is measured in energy units and needs a different conversion.
Last updated: 2026
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with an official source before making decisions.
Built and maintained by Vikas Dulgunde. Editorial standards.