What this calculator does
Fuel economy is measured two opposite ways round and that is the whole difficulty. Litres per 100 km is a consumption figure where lower is better. Miles per gallon and kilometres per litre are efficiency figures where higher is better. Converting between them is a division rather than a multiplication, so the numbers do not scale the way people expect.
The other trap is that there are two gallons. The US gallon is 3.785 litres and the imperial gallon is 4.546, about 20% larger, so the same car scores about 20% higher in imperial mpg than in US mpg. A car at 8 L/100km is 29.4 mpg US and 35.3 mpg imperial, and neither figure is wrong.
The formula
Everything converts through litres per 100 km. Kilometres per litre is 100 divided by that figure, which is why the relationship is inverse rather than proportional. US miles per gallon is 235.215 divided by the L/100km figure and imperial is 282.481 divided by it, with the two constants differing only because the two gallons differ.
| Term | Meaning |
|---|---|
| L/100km | Litres consumed per 100 kilometres. A consumption measure, so lower is better. |
| km/L | Kilometres travelled per litre. An efficiency measure, so higher is better. |
| mpg (US) | Miles per US gallon of 3.785 litres. |
| mpg (imperial) | Miles per imperial gallon of 4.546 litres, about 20% larger than the US gallon. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The fuel economy figure. |
| Unit | Which measure it is in. Check whether an mpg figure is US or imperial, since they differ by about 20%. |
When to use it
Comparing cars sold in different markets
A US review quoting mpg and a local specification quoting L/100km cannot be compared directly, and the US and imperial gallons add a second layer of difference on top.
Estimating fuel for a trip
The fuel for 500 km figure turns an economy rating into a quantity you can price, which is usually the question behind the conversion.
Judging whether an improvement is worthwhile
Because the scales are inverse, equal steps in mpg are not equal savings. Going from 15 to 20 mpg saves far more fuel than going from 40 to 45, which is much clearer in L/100km.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What do common fuel economy figures convert to?
A range of consumption figures from an efficient small car to a large vehicle.
| L/100 km | Kilometres per litre | Miles per gallon (US) | Miles per gallon (imperial) | Fuel for 500 km |
|---|---|---|---|---|
| 4 L/100km | 25.00 | 58.80 | 70.62 | 20.0 litres |
| 6 L/100km | 16.67 | 39.20 | 47.08 | 30.0 litres |
| 8 L/100km | 12.50 | 29.40 | 35.31 | 40.0 litres |
| 10 L/100km | 10.00 | 23.52 | 28.25 | 50.0 litres |
| 15 L/100km | 6.67 | 15.68 | 18.83 | 75.0 litres |
Questions
How do I convert mpg to L/100km?
Divide 235.215 by the US mpg figure, or 282.481 by the imperial figure. It is a division rather than a multiplication because the two measures run in opposite directions, so a high mpg gives a low L/100km.
Why are there two different mpg figures?
Because the US and imperial gallons are different volumes: 3.785 litres against 4.546. The imperial gallon is about 20% larger, so a car travels about 20% further on one, and its imperial mpg figure is correspondingly higher for identical real-world economy.
Which measure is better to use?
Litres per 100 km, for comparing improvements. Because it is a consumption measure it scales linearly with the fuel actually used, so a change of 1 L/100km means the same amount of fuel whether the car is efficient or thirsty. A change of 1 mpg does not.
Why does an mpg improvement matter less on an efficient car?
Because of the inverse relationship. Going from 15 to 20 mpg saves about 1.7 gallons over 100 miles; going from 40 to 45 saves about 0.28 gallons over the same distance. The mpg step is the same size and the fuel saved differs by a factor of six.
For the litre and gallon relationships underneath these figures, see the volume converter. For the distance side of the same calculation, see the length converter.