Fuel Economy Calculator
Convert between different fuel economy units and optimize your vehicle's efficiency.
Why a Fuel Economy Converter Is Needed
A fuel economy converter translates a vehicle's efficiency between the different units used around the world. The same car can be described as 30 mpg in the United States, 36 mpg in the United Kingdom, 12.75 km/l, or 7.84 L/100km, and these all mean the same real performance. Without a converter, comparing cars across markets or reading specs from abroad is confusing.
There are two families of units. Distance-per-fuel units like miles per gallon (mpg) and kilometres per litre (km/l) get bigger as efficiency improves. Fuel-per-distance units like litres per 100 kilometres (L/100km) get smaller as efficiency improves. Because they move in opposite directions, converting between them requires inversion, not just multiplication, which is exactly what the tool handles for you.
The most important catch is that the US and UK gallon are not the same size. The UK (imperial) gallon is about 4.546 litres, while the US gallon is about 3.785 litres. This means UK mpg figures look about 20 percent higher than US mpg for an identical car, so always check which gallon a number uses.
The Conversion Formulas
Each conversion follows a fixed formula. These are exact unit conversions, not estimates, since they are based on defined volumes and distances.
- L/100km = 235.215 / US mpg
- L/100km = 282.481 / UK mpg
- US mpg = 235.215 / L/100km
- km/l = US mpg x 0.42514 (or 100 / L/100km)
- UK mpg = US mpg x 1.20095
The constants come from the underlying units: 235.215 combines miles to kilometres and US gallons to litres, while 282.481 does the same using the larger imperial gallon. The table below shows the same handful of cars expressed every way, so you can see how the units relate.
| US mpg | UK mpg | km/l | L/100km |
|---|---|---|---|
| 20 | 24.0 | 8.5 | 11.76 |
| 30 | 36.0 | 12.75 | 7.84 |
| 40 | 48.0 | 17.0 | 5.88 |
| 50 | 60.0 | 21.25 | 4.70 |
Notice that doubling mpg does not halve fuel use in a linear, intuitive way across the whole range, which is one reason much of the world prefers L/100km for comparing efficiency directly.
Reading the Results and Avoiding Mistakes
Fuel economy figures are precise as unit conversions, but real-world fuel use always depends on driving conditions. The converter tells you that 30 US mpg equals 7.84 L/100km exactly; it does not promise your car will hit that number in traffic, in cold weather, or fully loaded.
A few practical points keep your comparisons honest:
- Confirm the gallon. A figure quoted simply as "mpg" is ambiguous between US and UK gallons until you know the source. UK figures are roughly 20 percent higher for the same car.
- Lower L/100km is better. Unlike mpg, smaller numbers mean a more efficient vehicle, which can feel counterintuitive at first.
- Test cycles differ. Official figures come from standardised tests (such as WLTP or EPA) that often differ from everyday driving, so two countries' stickers may not be directly comparable even after unit conversion.
- Watch electric efficiency separately. EVs are measured in kWh per 100 km or miles per kWh, which these fuel formulas do not cover.
Used carefully, a fuel economy converter lets you compare any car against any other on a single scale, decode foreign specifications, and understand exactly how much fuel a journey will use whichever unit you started from.
Frequently Asked Questions
For US mpg, divide 235.215 by the mpg value. For UK mpg, divide 282.481 instead, because the imperial gallon is larger. For example, 30 US mpg equals about 7.84 L/100km, while 30 UK mpg equals about 9.42 L/100km.
Because the gallons differ in size. The imperial (UK) gallon is about 4.546 litres and the US gallon is about 3.785 litres. The same fuel use therefore gives a UK mpg figure roughly 20 percent higher than the US mpg figure for an identical car.
Lower is better. L/100km measures how much fuel is used to travel a fixed distance, so a smaller number means the car uses less fuel. This is the opposite of mpg and km/l, where higher numbers indicate better efficiency.
Multiply US mpg by about 0.42514 to get km/l, or compute 100 divided by the L/100km value. For example, 30 US mpg is about 12.75 km/l. Use the UK conversion path if your mpg figure is based on the imperial gallon.
The unit conversions themselves are exact, based on defined volumes and distances. What varies is real-world fuel use, which depends on driving style, load, weather, and traffic. So the math is precise, but no car matches its rated figure in all conditions.
L/100km expresses fuel consumption per fixed distance, which makes it easy to compare cars and estimate trip costs directly. Many people find it more intuitive than mpg because the number maps straight to how much fuel a journey of a known length will require.