To convert US miles per gallon to liters per 100 km, divide 235.215 by the mpg. So 30 mpg is 235.215 ÷ 30 = 7.84 L/100km, and 40 mpg is 5.88 L/100km. The reverse uses the same number: mpg = 235.215 ÷ L/100km. For UK (imperial) mpg, use 282.481 instead.
Why do you divide instead of multiply? #
Most conversions multiply by a factor. This one can’t, because the two units measure opposite things:
- mpg is distance per unit of fuel. Higher is better.
- L/100km is fuel per unit of distance. Lower is better.
One is the reciprocal of the other, so the conversion is a division. The 235.215 comes from putting the exact definitions together: 100 × 3.785411784 L per gallon ÷ 1.609344 km per mile = 235.2146. NIST SP 811 gives the same rule: “divide 235.215 by number of miles per gallon.”
For UK mpg, the imperial gallon is 4.54609 L, which gives 100 × 4.54609 ÷ 1.609344 = 282.481.
km/L, used in Japan, India and much of Latin America, is a plain multiplication: km/L = US mpg × 0.425144.
US mpg to L/100km chart #
| US mpg | L/100km | km/L | Same car in UK mpg |
|---|---|---|---|
| 10 | 23.52 | 4.25 | 12.0 |
| 15 | 15.68 | 6.38 | 18.0 |
| 20 | 11.76 | 8.50 | 24.0 |
| 22 | 10.69 | 9.35 | 26.4 |
| 25 | 9.41 | 10.63 | 30.0 |
| 28 | 8.40 | 11.90 | 33.6 |
| 30 | 7.84 | 12.75 | 36.0 |
| 32 | 7.35 | 13.60 | 38.4 |
| 35 | 6.72 | 14.88 | 42.0 |
| 40 | 5.88 | 17.01 | 48.0 |
| 45 | 5.23 | 19.13 | 54.0 |
| 50 | 4.70 | 21.26 | 60.0 |
| 60 | 3.92 | 25.51 | 72.1 |
L/100km to mpg chart #
| L/100km | US mpg | UK mpg | km/L |
|---|---|---|---|
| 3 | 78.4 | 94.2 | 33.33 |
| 4 | 58.8 | 70.6 | 25.00 |
| 5 | 47.0 | 56.5 | 20.00 |
| 5.5 | 42.8 | 51.4 | 18.18 |
| 6 | 39.2 | 47.1 | 16.67 |
| 6.5 | 36.2 | 43.5 | 15.38 |
| 7 | 33.6 | 40.4 | 14.29 |
| 8 | 29.4 | 35.3 | 12.50 |
| 9 | 26.1 | 31.4 | 11.11 |
| 10 | 23.5 | 28.2 | 10.00 |
| 12 | 19.6 | 23.5 | 8.33 |
| 15 | 15.7 | 18.8 | 6.67 |
Why are UK mpg figures higher than US ones? #
Because the UK gallon is bigger. An imperial gallon holds 4.546 L and a US gallon 3.785 L, so a car goes about 20% farther on a UK gallon. The same car rated at 40 US mpg is about 48 UK mpg.
That trips up anyone reading British car reviews in the US, or the other way round. A British hatchback “doing 50 mpg” is 50 ÷ 1.20095 = 41.6 US mpg, and 282.481 ÷ 50 = 5.65 L/100km. More on the two gallons in US gallon vs imperial gallon.
What is the MPG illusion? #
Because mpg is a reciprocal, equal mpg gains don’t save equal amounts of fuel. The EPA’s fuel economy label guide gives the example: over 1,000 miles, going from 10 to 15 mpg saves about 33 gallons, but going from 30 to 35 mpg saves only about 5 gallons. That’s why the US window sticker shows fuel consumption as well as mpg.
Scaled up to 12,000 miles a year:
| Upgrade | Fuel saved per year |
|---|---|
| 10 → 20 mpg | 600 gallons |
| 30 → 40 mpg | 100 gallons |
| 40 → 50 mpg | 60 gallons |
Replacing a very thirsty vehicle saves far more fuel than trading an efficient car for a slightly more efficient one. L/100km and gallons per 100 miles show this directly, because they’re proportional to the fuel you actually buy.
Worked examples #
A European car is rated at 5.5 L/100km. 235.215 ÷ 5.5 = 42.8 US mpg.
Your pickup averages 20 US mpg and a Canadian friend asks what that is. 235.215 ÷ 20 = 11.76 L/100km.
A Japanese spec sheet lists 18 km/L. 18 ÷ 0.425144 = 42.3 US mpg, and 100 ÷ 18 = 5.56 L/100km.
You want gallons per 100 miles. Divide 100 by the mpg. 25 mpg is 4 gallons per 100 miles.
Converted ratings still aren’t directly comparable #
A converted number is only as comparable as the tests behind it. US EPA ratings and European WLTP figures come from different test cycles, so a European car’s official L/100km, converted to mpg, won’t necessarily match what the same car would get on the EPA test. Converting fixes the units, not the method. For your own car, the most honest figure is what you log at the pump.
Converting fuel economy on your phone #
Convert Units Calculator has a Fuel category in version 1.0.5 with L/100km, km/L, US mpg and UK mpg, and it handles the reciprocal math for you, so you type the number you have and read the others. It uses the exact gallon and mile definitions, which is why its answers match the tables above. It works offline, which helps when you’re reading a rental-car spec sheet abroad.
For the underlying volume and distance conversions, see liters to gallons and km to miles.
Frequently asked questions #
What is 30 mpg in L/100km? #
30 US mpg is 7.84 L/100km. 30 UK mpg is 9.42 L/100km.
What is 6 L/100km in mpg? #
6 L/100km is 39.2 US mpg or 47.1 UK mpg.
How do I convert km/L to mpg? #
Multiply km/L by 2.352 for US mpg, or by 2.825 for UK mpg. So 15 km/L is 35.3 US mpg.
Is lower L/100km better? #
Yes. L/100km measures how much fuel the car uses to cover 100 km, so a smaller number means less fuel. With mpg it’s the other way round: higher is better.
Why doesn’t the calculator just multiply? #
Because mpg and L/100km are reciprocals. Doubling mpg halves L/100km, so there’s no single factor that works at every value. You have to divide a constant (235.215 for US, 282.481 for UK) by the number you have.