Litres per 100 kilometres and miles per gallon are inverses of each other, which is why they behave so differently. Fuel consumed per unit of distance rises linearly with a worse engine, so L/100 km can be added, averaged and scaled sensibly. Miles per gallon is distance per unit of fuel and is a reciprocal, which is why going from 15 to 20 mpg saves far more fuel than going from 50 to 55.
The conversion constant for imperial figures is 282.481, and it is not arbitrary: it is 100 x 4.54609 / 1.609344, the litres in an imperial gallon divided by the kilometres in a mile, scaled to 100 km. For American figures the constant is 235.215, from the same expression with a 3.785411784 litre US gallon. Divide the constant by the mpg figure, or divide it by the L/100 km figure to go back the other way.
So 45 mpg imperial is 282.481 ÷ 45 = 6.277 litres per 100 km, which is also 15.93 kilometres per litre and 37.47 mpg in US units. On a 50 litre tank that is a theoretical range of 797 km. The reciprocal relationship is why small improvements at the thirsty end of the scale are worth chasing hardest.
The formula
282.481- 100 × litres per imperial gallon ÷ km per mile
235.215- The same constant built from the 3.785411784 litre US gallon
km/L- Kilometres per litre — simply 100 ÷ the L/100 km figure
tank- Usable tank capacity, used for the range figure
How to use this calculator
- Enter the economy figure you already have.
- Choose the unit that figure is expressed in.
- Enter your tank size if you want the range estimate.
- The gauge shows litres per 100 km; the equivalents in other units sit below.
Worked examples
45 mpg imperial converted
282.481 ÷ 45 = 6.277 litres per 100 km. The same car is 37.47 mpg US and 15.93 km per litre. On a 50 litre tank the theoretical range is 797 km, or 495 miles.
Why the reciprocal matters
Going from 15 to 20 mpg imperial cuts consumption from 18.83 to 14.12 L/100 km — a saving of 4.71 litres every 100 km. Going from 50 to 55 mpg only saves 0.51 litres over the same distance, despite being the same five-mpg step.
How to read your score
Frequently asked questions
Where does the 282.481 constant come from?
It is 100 x 4.54609 / 1.609344. An imperial gallon holds 4.54609 litres and a mile is 1.609344 kilometres, so converting miles-per-gallon into litres-per-100-kilometres collapses into that single number. Nothing is rounded away until the final display.
Why is the US constant different?
Because a US liquid gallon is 3.785411784 litres, about 83 per cent of an imperial gallon. Running the same expression with that value gives 235.215. Using the imperial constant on a US figure overstates efficiency by roughly 20 per cent.
Which unit should I actually use?
Litres per 100 km if you are comparing running costs, because it scales linearly with distance and money. Miles per gallon is more intuitive for range. The two carry identical information; only the arithmetic behaves differently.
Is km per litre the same as L/100 km inverted?
Yes, exactly: km per litre is 100 divided by the L/100 km figure. It is common in parts of Asia and South America, and it behaves like mpg — a reciprocal measure where equal steps are not equally valuable.
Does the range figure account for the reserve?
No, it is the theoretical full-tank range using the whole quoted capacity. In practice the last couple of litres are not reliably drawable, so subtract a small reserve before planning a long leg around the number.
Economy conversion reference
| mpg (imp) | L/100km | mpg (US) | km per litre | Miles per litre |
|---|---|---|---|---|
| 20 mpg | 14.12 | 16.65 | 7.08 | 4.40 |
| 25 mpg | 11.30 | 20.82 | 8.85 | 5.50 |
| 30 mpg | 9.42 | 24.98 | 10.62 | 6.60 |
| 35 mpg | 8.07 | 29.14 | 12.39 | 7.70 |
| 40 mpg | 7.06 | 33.31 | 14.16 | 8.80 |
| 45 mpg | 6.28 | 37.47 | 15.93 | 9.90 |
| 50 mpg | 5.65 | 41.63 | 17.70 | 11.00 |
| 60 mpg | 4.71 | 49.96 | 21.24 | 13.20 |
| 70 mpg | 4.04 | 58.29 | 24.78 | 15.40 |
| 80 mpg | 3.53 | 66.61 | 28.32 | 17.60 |
Built from the exact constants 4.54609 L per imperial gallon, 3.785411784 L per US gallon and 1.609344 km per mile.