How to use the Fuel Economy Converter (MPG, km/L, L/100km)
- Enter the fuel economy figure you have.
- Choose the unit you are converting from and the unit you want.
- Read the converted figure and the sentence showing both values.
- Remember that for L/100 km a lower number is better, while for mpg and km/L a higher number is better.
Formula used
Everything is converted through litres per 100 km: L/100km = 235.215 ÷ mpg (US) = 282.481 ÷ mpg (UK) = 100 ÷ km/L. Miles per gallon and km/L go up as economy improves; L/100 km goes down.
Worked example
Here is the Fuel Economy Converter (MPG, km/L, L/100km) worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| Value | 30 |
| From | Miles per gallon (US) |
| To | Litres per 100 km |
| Result | |
| Converted value | 7.8404861 |
| In words | 30 mpg (US) = 7.8405 L/100 km |
Quick reference table
Common values converted with the units selected above.
| From | To |
|---|---|
| 10 mpg | 23.521458 l100 |
| 20 mpg | 11.760729 l100 |
| 25 mpg | 9.4085833 l100 |
| 30 mpg | 7.8404861 l100 |
| 40 mpg | 5.8803646 l100 |
| 50 mpg | 4.7042917 l100 |
Two directions of measurement
Miles per gallon and kilometres per litre measure distance for a given amount of fuel, so higher is better. Litres per 100 km measures fuel for a given distance, so lower is better. They are linked by reciprocals: L/100 km = 235.215 ÷ mpg (US) = 282.481 ÷ mpg (UK) = 100 ÷ km/L.
For example, 30 mpg (US) is 7.84 L/100 km, and 40 mpg (US) is 5.88 L/100 km. Going from 30 to 40 mpg cuts fuel use by 1.96 L per 100 km.
US mpg versus UK mpg
The UK gallon is larger than the US gallon, so the same car has a higher UK figure. A car rated at 30 mpg in the US is 36 mpg in UK terms. Conversely, 40 mpg (UK) is 33.3 mpg (US). Always check which gallon a quoted figure uses.
In km/L, 30 mpg (US) is 12.75 km/L, and a car that does 20 km/L uses 5 L/100 km.
Why mpg can mislead
Gains at low mpg save more fuel than the same gain at high mpg. Improving from 10 to 20 mpg (US) saves 11.8 L per 100 km, but improving from 40 to 50 mpg saves only 1.18 L per 100 km, although both are a gain of 10 mpg. This is why many experts prefer L/100 km for comparing efficiency.
Real-world fuel economy also depends on speed, load, tyre pressure, traffic and driving style, so the official figures are for comparison, not a guarantee.
Estimating fuel cost
To estimate trip cost, divide the distance by the fuel economy to get the fuel needed, then multiply by the price of fuel. The fuel cost calculator on this site does this for you. For example, a 500 km trip at 8 L/100 km uses 40 litres.
Electric vehicles use different units, such as kWh per 100 km or miles per gallon equivalent (MPGe), which are not covered by this converter.
Frequently asked questions
Why do US and UK MPG differ?
The UK gallon is larger (4.546 L) than the US gallon (3.785 L), so the same car has a higher MPG figure in the UK.
What is a good L/100 km?
Small modern cars are often around 5 to 6 L/100 km. Larger vehicles use more. A lower number is better.
How do I convert mpg to L/100 km?
Divide 235.215 by the mpg (US). For UK mpg, divide 282.481.
How do I convert km/L to L/100 km?
Divide 100 by the km/L figure.
Why is L/100 km better for comparing cars?
It is directly proportional to fuel used, so differences add up in a straightforward way.
Does this work for electric cars?
No. Electric efficiency uses kWh per 100 km or MPGe, which are not included.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.