Why the two units run backwards
Fuel consumption in Europe is written as litres per 100 kilometres. In the United States and the United Kingdom it is written as miles per gallon. The two numbers do not move together: when one goes up, the other goes down. That is because one measures fuel per distance and the other measures distance per fuel.
This is why a car that looks good at 5 L/100 km looks terrible when you see 47 mpg and great when you see 56 mpg UK. Same car, two gallons, two different numbers.
There is no single factor that converts between the two, because the relationship is inverse. Multiplying by a constant only works in one direction and only at one point. The correct formulas are:
- mpg (US) = 235.214583 ÷ L/100 km
- mpg (UK) = 282.4809363 ÷ L/100 km
- L/100 km = 235.214583 ÷ mpg (US), or 282.4809363 ÷ mpg (UK)
The constants come from unit definitions: one US gallon is 3.785411784 litres and one UK gallon is 4.54609 litres, and one mile is 1.609344 kilometres.
The exact formulas
The table below is a quick check. Values are rounded to one decimal place; use the converter for exact numbers.
| L/100 km | mpg (US) | mpg (UK) |
|---|---|---|
| 4.0 | 58.8 | 70.6 |
| 5.0 | 47.0 | 56.5 |
| 6.0 | 39.2 | 47.1 |
| 7.0 | 33.6 | 40.4 |
| 8.0 | 29.4 | 35.3 |
| 10.0 | 23.5 | 28.2 |
| 12.0 | 19.6 | 23.5 |
A quick reference table
The common mistake is treating the conversion as a factor: "7 L/100 km is about 33 mpg, so 3.5 L/100 km must be about 16.5 mpg". That is wrong. Halving consumption doubles mpg. 3.5 L/100 km is about 67 mpg US, not 16.5.
If you are comparing cars, remember that the gaps at the efficient end matter more in absolute fuel than the same gap at the wasteful end. Going from 5 to 4 L/100 km saves more fuel per distance than going from 15 to 14.
The mistake almost everyone makes
Consumption becomes money once you add distance and price. Litres used = distance in km × L/100 km ÷ 100. Multiply by the price per litre for the total. With miles and gallons, convert distance to km first or gallons to litres, and keep the units consistent.
For a 450 km trip at 6.5 L/100 km and 1.65 per litre, the car uses 29.25 litres and the fuel costs 48.26. A round trip doubles both. Our trip cost calculator does this with km or miles, four consumption units and three price units.
From consumption to trip cost
There is no universal number. A small petrol car might average 5-6 L/100 km (about 39-47 mpg US). A larger SUV might be 9-12 L/100 km (20-26 mpg US). The right comparison is between cars you are actually choosing, on the same cycle, not against a headline figure.
Two practical tips. First, use the same unit for every car you compare; switching units mid-comparison is where mistakes creep in. Second, treat published consumption as a best case. Short trips, cold engines, air conditioning and roof boxes all push real numbers up.
What counts as good fuel economy?
If you want to check a specific conversion, paste the value into the unit converter and choose Fuel economy. If you want the money, open the trip fuel cost calculator. Both run entirely in your browser.
Frequently asked
Is 7 L/100 km good?
It is respectable for a petrol car. Seven litres per 100 km is about 33.6 mpg US or 40.4 mpg UK, close to the combined figure of many mid-size cars.
Why are US and UK mpg different?
A UK gallon is 4.546 litres and a US gallon is 3.785 litres, so the same consumption always shows a higher mpg number in the UK. The constants differ for the same reason.
How do I convert km/L to L/100 km?
Divide 100 by the km/L value. Ten km/L becomes 10 L/100 km; 20 km/L becomes 5 L/100 km. It is the inverse relationship again, not a fixed factor.