What fuel economy is and why the unit keeps changing
Fuel economy measures how much petrol or diesel a vehicle burns to cover a given distance. The catch is that every region writes it differently: most of Europe and Latin America use liters per 100 km (L/100km), many countries prefer kilometers per liter (km/L), the United States reports miles per gallon (MPG US), and the United Kingdom uses its larger imperial gallon for MPG UK. The exact same efficiency can be spelled four ways that look unrelated.
This converter takes a value in any one of those four units and instantly shows the other three, plus a reference table. Everything runs inside your browser.
The key twist: L/100km is an inverse scale
This is the part that trips almost everyone up. With km/L, MPG US and MPG UK, a higher number means a more efficient car — it travels farther on the same amount of fuel. But L/100km works the opposite way: it counts how much fuel you burn over a fixed distance, so fewer liters is better. A car rated at 4 L/100km is far more efficient than one at 12 L/100km.
That’s why you can’t always convert with a plain multiplication. Moving from a direct scale to the inverse one requires dividing.
The formulas
Using L/100km as the base:
- km/L = 100 ÷ (L/100km)
- MPG (US) = 235.214583 ÷ (L/100km)
- MPG (UK) = 282.480936 ÷ (L/100km)
The constants come from combining the gallon (3.785411784 L for the US one, 4.54609 L for the imperial) with the mile (1.609344 km). MPG UK always reads higher than MPG US for the same car, because the imperial gallon is about 20% larger.
| L/100km | km/L | MPG (US) | MPG (UK) |
|---|---|---|---|
| 4 | 25 | 58.8 | 70.62 |
| 5 | 20 | 47.04 | 56.5 |
| 6 | 16.67 | 39.2 | 47.08 |
| 7 | 14.29 | 33.6 | 40.35 |
| 8 | 12.5 | 29.4 | 35.31 |
| 10 | 10 | 23.52 | 28.25 |
| 12 | 8.33 | 19.6 | 23.54 |
| 15 | 6.67 | 15.68 | 18.83 |
Worked example
Your car uses 8 L/100km and you want to compare it with a US spec sheet in MPG:
- km/L = 100 ÷ 8 = 12.5 km/L
- MPG (US) = 235.214583 ÷ 8 = 29.40 MPG
- MPG (UK) = 282.480936 ÷ 8 = 35.31 MPG
So 8 L/100km works out to roughly 29 US MPG — a typical figure for a compact petrol sedan.
How to use the converter
- Type the economy figure into the Value to convert field.
- Pick the matching source unit from the dropdown.
- The other three units appear immediately in the equivalents panel.
- Hit Copy to grab the result, or read off round values from the table.
Frequently asked questions
Why is UK MPG higher than US MPG?
Because the British imperial gallon (4.546 L) is larger than the US gallon (3.785 L). More liters per gallon means more miles covered per gallon, even though the real efficiency is identical. Always check which gallon a spec sheet is using.
Is a high L/100km number good?
No, it’s the reverse. L/100km measures how much fuel you burn to travel 100 km, so lower is better. A value of 5 L/100km is very efficient; 14 L/100km describes a thirsty vehicle.
How do I convert km/L to L/100km?
Divide 100 by the km/L figure. For example, 20 km/L equals 100 ÷ 20 = 5 L/100km. The relationship is reciprocal both ways: 100 ÷ 5 also gives 20 km/L.
Does it work for motorbikes, trucks or electric vehicles?
For motorbikes and trucks, yes — as long as the economy is measured in liquid fuel. Electric vehicles are rated in kWh per 100 km or miles per kWh, which are energy figures and don’t convert with these formulas.