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Fuel Economy Converter
Compare a car rated in miles per gallon with one rated in litres per 100 km. US and imperial gallons are separate choices because they differ by about 20%, and the inverse relationship is handled for you.
Distance per fuel versus fuel per distance
Two different ideas share the name fuel economy. Miles per gallon and kilometres per litre measure distance per unit of fuel, so larger is better. Litres per 100 km measures fuel per distance, so smaller is better. The two are reciprocals, which is why converting between them is not a multiplication.
The anchor constants come from exact definitions: with 1 mile = 1.609344 km and 1 US gallon = 3.785411784 L, L/100 km = 235.2146 ÷ mpg (US). With the imperial gallon of 4.54609 L, L/100 km = 282.4809 ÷ mpg (imperial). For kilometres per litre, L/100 km = 100 ÷ (km/L).
US and imperial mpg are not the same
An imperial gallon is about 20% larger than a US gallon, so the same car has a higher imperial figure. A vehicle rated 30 mpg (US) is about 36 mpg (imperial), or 7.84 L/100 km. Quoting only "mpg" without a country is a common cause of apparent differences between reviews and brochures.
| L/100 km | mpg (US) | mpg (imperial) |
|---|---|---|
| 5 | 47.0 | 56.5 |
| 8 | 29.4 | 35.3 |
| 10 | 23.5 | 28.2 |
| 12 | 19.6 | 23.5 |
A trap when averaging
Do not average mpg figures directly. If a car does 20 mpg for half the distance and 40 mpg for the other half, the overall figure is not 30 mpg. For 100 miles of each, fuel use is 5 + 2.5 = 7.5 gallons over 200 miles, which is 26.7 mpg. Averaging in litres per 100 km is safer because fuel adds directly.
The same inverse effect makes small gains in poor economy worth more than big gains in good economy. Improving from 10 to 20 mpg saves more fuel over a distance than improving from 40 to 50 mpg.
Reading published figures
Official ratings come from standardized test procedures, and different regions use different tests, so a rating from one scheme can differ from another for the same car. Real-world results also vary with speed, load, temperature, tyre pressure and driving style. Use the converter to compare numbers on the same basis, and treat any single figure as an estimate rather than a promise.
To estimate running costs, multiply the litres per 100 km by the fuel price per litre. A car using 7.84 L/100 km with fuel at 2.00 per litre costs about 15.68 per 100 km, or 0.157 per kilometre. The same calculation works in any currency, because the price is an input you supply.
Worked example
A US car is rated at 30 miles per US gallon. How many litres does it use per 100 kilometres?
Inputs
| Fuel economy | 30 |
|---|---|
| From | Miles per US gallon (mpg) |
| To | Litres per 100 km (L/100 km) |
Result
30 mpg (US) = 7.840486 L/100 km
| Miles per imperial gallon (mpg (imp)) | 36.028498 |
|---|---|
| Kilometres per litre (km/L) | 12.754311 |
- Miles per gallon and km per litre rise as a car gets more efficient. Litres per 100 km falls. The relationship is an inverse, so the conversion is not a simple multiplication.
Accuracy note: Factors use the standard definitions stated on this page. Results are rounded for display; for engineering or legal work use the full-precision factors.
Frequently asked questions
How do I convert mpg to L/100 km?
Divide 235.2146 by the US mpg figure. For example, 30 mpg (US) gives 235.2146 ÷ 30 = 7.84 L/100 km. For imperial mpg use 282.4809 instead.
Which is better, higher mpg or lower L/100 km?
Higher mpg is better, because it means more distance per unit of fuel. Lower L/100 km is better, because it means less fuel per distance.
Why is imperial mpg higher than US mpg?
An imperial gallon is 4.54609 litres and a US gallon is 3.785411784 litres. The same distance uses fewer imperial gallons, so the imperial number is about 20% higher.
How many km per litre is 10 L/100 km?
It is 10 km per litre, found by dividing 100 by 10.
Can I use this for electric vehicles?
No. This tool covers liquid fuel. Electric consumption is quoted in kWh per 100 km or miles per kWh, which need their own conversion.