When is a fuel economy converter useful?
A US car review quotes miles per gallon, a German spec sheet quotes liters per 100 kilometers, and a Japanese brochure quotes kilometers per liter. None of those numbers can be compared by eye, and the two "mpg" figures are not even the same size. This converter takes one value and shows all four common notations side by side, with a one-line trace of the arithmetic so you can check it. It is most handy when you are looking at a car imported from another market, reading an overseas road test, or trying to make sense of a used-car listing that uses a unit you do not normally think in.
Why the units differ, and which way is "better"
km/L and mpg measure distance per unit of fuel. They are efficiency figures, so a bigger number means a thriftier car. L/100km measures fuel per unit of distance. It is a consumption figure, so a smaller number means a thriftier car. Mixing the two directions is the single most common mistake: a car rated 5 L/100km is not "worse" than one rated 8 L/100km, it is the more economical of the two. If you are ever unsure, convert everything to a single unit before ranking.
There is also a practical reason some regions prefer consumption figures. Fuel cost scales with litres burned, and litres burned scale with distance driven. Consumption in L/100km therefore adds up naturally: halve the consumption and you halve the fuel bill for the same trip. Efficiency figures such as mpg do not add up that way, which is why improvements from a very thirsty vehicle are easy to underestimate. Going from 10 mpg to 20 mpg saves far more fuel over a given distance than going from 30 mpg to 40 mpg, even though both are a 10 mpg gain.
Conversion formulas and constants
- km/L to L/100km: 100 / (km/L). The reverse direction is the same formula, because the two are reciprocals.
- mpg (US) to km/L: mpg x 1.609344 / 3.785411784.
- mpg (UK) to km/L: mpg x 1.609344 / 4.54609.
- Defined constants: 1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 imperial (UK) gallon = 4.54609 L.
- Handy shortcut: mpg (US) x L/100km is about 235.2, and mpg (UK) x L/100km is about 282.5.
The imperial gallon is about 20% larger than the US gallon (4.54609 / 3.785411784 is roughly 1.201), so the same car always has a bigger UK mpg figure than US mpg figure. A listing that simply says "mpg" is ambiguous until you know which country the number came from, which is why this tool lets you pick either gallon as the input and shows both in the output.
Reference table: starting from km/L
| km/L | L/100km | mpg (US) | mpg (UK) |
|---|---|---|---|
| 5 | 20.00 | 11.8 | 14.1 |
| 8 | 12.50 | 18.8 | 22.6 |
| 10 | 10.00 | 23.5 | 28.2 |
| 12 | 8.33 | 28.2 | 33.9 |
| 15 | 6.67 | 35.3 | 42.4 |
| 17 | 5.88 | 40.0 | 48.0 |
| 20 | 5.00 | 47.0 | 56.5 |
| 25 | 4.00 | 58.8 | 70.6 |
Every row above is computed when the page is built, using the formulas and constants listed earlier.
Reference table: starting from L/100km
| L/100km | km/L | mpg (US) | mpg (UK) |
|---|---|---|---|
| 4 | 25.00 | 58.8 | 70.6 |
| 5 | 20.00 | 47.0 | 56.5 |
| 6 | 16.67 | 39.2 | 47.1 |
| 7 | 14.29 | 33.6 | 40.4 |
| 8 | 12.50 | 29.4 | 35.3 |
| 10 | 10.00 | 23.5 | 28.2 |
| 12 | 8.33 | 19.6 | 23.5 |
Worked examples
- A US listing says 30 mpg. 30 x 1.609344 / 3.785411784 is about 12.75 km/L. Taking the reciprocal, 100 / 12.75 is about 7.84 L/100km. If the same number came from a UK source it would mean a noticeably better car: 30 UK mpg is about 10.62 km/L, or roughly 9.4 L/100km.
- A European spec sheet says 6.5 L/100km. 100 / 6.5 is about 15.38 km/L. In US miles per gallon that is roughly 36.2 mpg, and in UK miles per gallon roughly 43.5 mpg.
- A British road test says 45 mpg. Because it is almost certainly an imperial gallon figure, 45 x 1.609344 / 4.54609 is about 15.93 km/L, or 6.28 L/100km. Reading it as US mpg would give about 19.13 km/L, making the car look roughly 20% more efficient than it really is.
- Averaging two stretches of road. Suppose you drive 100 km at 10 km/L and then another 100 km at 20 km/L. The average is not 15 km/L. You burned 10 L plus 5 L, which is 15 L for 200 km, so the true average is about 13.33 km/L. The correct formula is total distance divided by total fuel.
Common mistakes and limits
- The two kinds of unit do not convert with one multiplier: km/L and mpg are proportional to each other (1 km/L is about 2.352 US mpg or 2.825 UK mpg), but L/100km is the reciprocal of both, so no constant factor turns it into either. Use the formulas above rather than a rule of thumb.
- Fuel economy is averaged by fuel and distance, not by taking the arithmetic mean of two ratings. See the example above for how far off a plain average can be.
- Matching units does not match test methods. Official ratings come from different laboratory procedures in different regions, with different speed profiles, temperatures and rules for accessories, so a car's US figure and its European figure can differ even after perfect conversion. This tool converts units only; it does not translate one test cycle into another.
- Electric vehicles are out of scope. Their efficiency is expressed in kWh per distance or in an energy-equivalent figure such as MPGe, which compares energy content rather than liquid volume. Do not feed an MPGe value into a liters-based converter.
- Results are rounded for display to two decimals. Converting back and forth repeatedly can accumulate tiny rounding differences.
- Diesel and gasoline contain different amounts of energy per liter, so a diesel car and a petrol car with the same km/L are not identical in energy use or in running cost per kilometer.
What changes fuel economy in real life
Once the units agree, the next question is how a number relates to your own driving. Stop-and-go traffic, cold-weather warm-up, air conditioning, low tire pressure, roof boxes and heavy loads all cost fuel. Steady cruising at moderate speed usually does best because aerodynamic drag grows quickly as speed rises. These effects can move a real-world result by more than the gap between two similar cars on a spec sheet, so a difference of under 1 km/L (or about 2 mpg) is rarely a good reason to prefer one car over another.
A useful habit is to convert efficiency into a monthly or yearly fuel quantity. If you drive 1,500 km a month in a car that returns 12 km/L, you burn 1,500 / 12 = 125 L. At an assumed price of 1.70 per liter (an illustrative figure, not a market quote; use your local price) that is about 212.50 a month. If the car only manages 10 km/L, it uses 150 L and costs 255.00, a difference of about 42.50 a month for the same driving. A comparison like this tells you far more than the unit itself.
How to use the result
Convert every candidate car to the one unit you think in, then compare. Next, multiply by your own yearly distance to turn the figure into litres and money. For the second step use thefuel efficiency calculator, which works out real economy from a fill-up. For other measurements, the unit convertercovers distance and volume, and the electricity cost calculatorhelps when you are weighing a plug-in or electric vehicle against a petrol one.
Frequently Asked Questions
Can this convert mpg based on the UK (imperial) gallon?
Yes. Choose "mpg - UK (imperial) gallon" as the input unit, or read the UK figure in the results. The imperial gallon (4.54609 L) is about 20% larger than the US gallon (3.785 L), so the same car shows a UK mpg number roughly 1.2 times its US mpg number.
Is a smaller L/100km value better?
Yes. L/100km is the amount of fuel needed to cover 100 km, so smaller means less fuel burned. km/L and mpg work the opposite way: bigger is better.
Why will the tool not convert zero or negative numbers?
A fuel economy of zero would mean burning fuel without moving, and zero L/100km would mean moving without fuel. Neither is physically meaningful, and dividing by zero is undefined, so the tool asks for a value greater than zero.
Why does my real fuel economy differ from the official rating?
Official ratings come from a prescribed test under controlled conditions. Real driving adds traffic, hills, weather, accessories, tire pressure, load and personal driving style. The converter changes only the unit; it cannot correct for those differences.
Does the same formula apply to hybrids and diesels?
Unit conversion is identical for any liquid fuel. When comparing running costs, though, apply each fuel's own price per litre, because petrol, diesel and LPG are priced differently. For plug-in hybrids, a single liquid-fuel figure leaves out the electricity used and can flatter the car.
What should I check first when comparing fuel economy figures from different sources?
Check the direction of the unit and the type of gallon. Then check that both numbers come from the same test procedure. Finally, remember that differences smaller than your real-world variation, roughly 1 km/L or 2 mpg, seldom matter.