Fuel Economy Converter
Convert fuel economy between US mpg, imperial mpg, km per litre and litres per 100 km, with the cost per mile and km when you add a fuel price.
Result
In litres per 100 km: 7.8405 L/100 km
Exact: the factors follow from the definitions of the mile and the gallons.
- In miles per US gallon
- 30 mpg
- In miles per imperial gallon
- 36.0285 mpg (imp)
- In kilometres per litre
- 12.7543 km/L
- Fuel per 100 miles
- 3.33 gal (12.62 L)
- Fuel per 100 km
- 7.84 L (2.07 gal)
https://calcgallery.com/notation-conversions/fuel-economy-converter-calculator/
Unit conversions follow from exact definitions and published notation rules. Sizes follow published standards and are approximate: makers grade their own, so check their chart. Figures are rounded only for display. Read the full disclaimer
How to use the Fuel Economy Converter
- Enter the fuel economy and choose its measure from the unit list beside it.
- Choose which measure to show as the headline; all four are listed.
- Add a fuel price, optional, for the fuel cost per mile and per kilometre.
The conversions
L/100 km = 235.215 ÷ US mpgL/100 km = 282.481 ÷ imperial mpgL/100 km = 100 ÷ km per litre235.215 is 100 × 3.785411784 ÷ 1.609344: a hundred kilometres’ worth of US gallons per mile. The imperial figure uses 4.54609 litres instead. Both follow exactly from the definitions of the mile and the gallons, so the conversion itself introduces no error.
Why the direction matters
Miles per gallon counts distance per unit of fuel; litres per 100 km counts fuel per unit of distance. Because one is the reciprocal of the other, equal steps in mpg are not equal savings. Going from 10 to 12 mpg saves more fuel than going from 40 to 60 mpg over the same distance. The curve in the diagram shows it: at low mpg, each extra mile per gallon cuts fuel use steeply; at high mpg, the curve is almost flat.
With your numbers
- L/100 km = 235.2146 ÷ US mpg = 282.4809 ÷ imperial mpg = 100 ÷ km/L
Quick reference
Exact: the factors follow from the definitions of the mile and the gallons.
| Example | Result |
|---|---|
| 30 US mpg | In litres per 100 km: 7.8405 L/100 km |
| 7.8 L/100 km | In miles per imperial gallon: 36.2155 mpg (imp) |
| 30 US mpg in km/L | In kilometres per litre: 12.7543 km/L |
| 10 L/100 km | In miles per US gallon: 23.5215 mpg |
| 6 L/100 km | In miles per imperial gallon: 47.0802 mpg (imp) |
| 50 US mpg | In litres per 100 km: 4.7043 L/100 km |
Common questions about the Fuel Economy Converter
- How do I convert mpg to litres per 100 km?
- Divide 235.215 by the US mpg figure, or 282.481 by an imperial mpg figure. 30 US mpg is 235.215 ÷ 30 = 7.84 L/100 km. The two constants come from the definitions: 100 km in miles, times litres in a gallon.
- Is a UK mpg the same as a US mpg?
- No. The imperial gallon is 4.546 litres and the US gallon 3.785, so the same car’s economy reads about 20% higher in imperial mpg. 30 imperial mpg is 9.42 L/100 km, while 30 US mpg is 7.84. Always check which gallon a figure uses before comparing.
- Why is L/100 km a better measure than mpg?
- Because it measures what you pay for: fuel used for a fixed distance. Over 10,000 miles, improving from 20 to 25 mpg saves 100 gallons, while improving from 40 to 50 mpg saves only 50, even though both are 25% gains. In litres per 100 km the saving is simply the difference between the two figures, so comparisons come out right without any thought.
- Why can the page not convert zero?
- The measures are reciprocals of each other: litres per 100 km is fuel over distance, mpg distance over fuel. Zero in one would be infinity in the other, so a zero economy is refused.