Fuel Economy Converter (MPG ↔ L/100km)
Convert between fuel-economy units in any direction. Enter a value in US MPG, UK MPG, L/100km, or km/L and get the other three, using exact gallon and mile conversions.
Example: with Value 30 · Convert from MPG (US gallon) → L/100km: 7.84 L/100km.
- MPG (US)30 mpg (US)
- km per liter12.75 km/L
- MPG (UK)36.03 mpg (UK)
Computed by the calculator below using its default values. Change any input to see your own numbers.
1 US gallon = 3.785411784 L and 1 mile = 1.609344 km. MPG and L/100km are inverses — as one rises the other falls — so you cannot average MPG figures directly.
Two ways of saying the same thing
The United States and the United Kingdom measure fuel economy as distance per fuel: miles per gallon, where higher is better. Most of the rest of the world measures fuel per distance: liters per 100 kilometers, where lower is better. They describe the same efficiency from opposite ends, which is why converting one to the other means taking a reciprocal, not just scaling.
That inverse relationship has a practical catch. If you drive 100 miles at 20 MPG and 100 miles at 40 MPG, your average is not 30 MPG — you burned 5 + 2.5 = 7.5 gallons over 200 miles, which is 26.7 MPG. To average economy, average the fuel used, not the MPG.
Mind the gallon
A US gallon is 3.785 liters; a UK (imperial) gallon is 4.546 liters, about 20 percent larger. So the same car rated 40 MPG in Britain is only about 33 MPG in the States, even though nothing changed but the gallon. Always check which gallon a figure uses before comparing cars across the Atlantic.
This tool keeps the mile and both gallons at their exact defined sizes, so round-trips stay consistent no matter which unit you start from.
How it’s calculated
Every input is converted through kilometers per liter. 1 mile = 1.609344 km and 1 US gallon = 3.785411784 L, so 1 US MPG = 0.425144 km/L; the UK gallon is 4.54609 L. Then L/100km = 100 / (km per L) and km/L = 100 / (L/100km), since the two are reciprocals.
Results use US gallons unless UK MPG is selected. Real-world economy varies with speed, load, terrain, and driving style, so treat any single figure as nominal.
Fuel economy converted
| Fuel economy (US) | L/100km | km/L |
|---|---|---|
| 15 mpg | 15.7 L/100km | 6.4 km/L |
| 25 mpg | 9.4 L/100km | 10.6 km/L |
| 30 mpg | 7.8 L/100km | 12.8 km/L |
| 40 mpg | 5.9 L/100km | 17.0 km/L |
| 50 mpg | 4.7 L/100km | 21.3 km/L |
Computed with 1 US gal = 3.785411784 L and 1 mi = 1.609344 km (NIST exact factors).
Common mistakes
- Averaging two MPG numbers to compare trips — because MPG and L/100km are inverse, average the fuel used instead.
- Mixing US and UK gallons; a UK gallon is about 20 percent larger, so 40 UK MPG is only about 33 US MPG.
- Reading L/100km as higher-is-better; it is the opposite of MPG, so a lower L/100km is more efficient.
- Rounding in the middle of the calculation rather than only at the end.
Frequently asked questions
How do I convert MPG to L/100km?
Convert MPG to km per liter (1 US MPG = 0.425144 km/L), then divide 100 by that. For example 30 US MPG is 12.75 km/L, and 100 / 12.75 is about 7.84 L/100km.
Why are MPG and L/100km inverse?
MPG is distance per unit of fuel and L/100km is fuel per unit of distance. One is the reciprocal of the other, so as MPG rises, L/100km falls.
What is the difference between US and UK MPG?
They use different gallons. A US gallon is 3.785 L and a UK gallon is 4.546 L, so a UK MPG figure is about 20 percent higher than the US figure for the same car.
Is a higher or lower number better?
For MPG and km/L, higher is more efficient. For L/100km, lower is more efficient because it means less fuel per distance.
What is km/L?
Kilometers per liter — how far you travel on one liter of fuel. It is the metric mirror of MPG, and higher is better.