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Fuel economy is quoted in incompatible ways around the world: miles per gallon (MPG) in the US and UK, litres per 100 km across Europe, and km/L in some countries. Comparing a car's efficiency across these takes more than a multiply — because MPG and L/100km are inversely related, higher is better for one and lower is better for the other.
Enter a value, pick the units, and the converted figure appears at once, along with the L/100km equivalent.
How is it calculated?
The units — and the inversion
- MPG (miles per gallon): distance per fuel — higher is better.
- L/100km (litres per 100 km): fuel per distance — lower is better.
- km/L: distance per fuel — higher is better.
Because MPG measures distance-over-fuel and L/100km measures fuel-over-distance, they're reciprocals. You can't add or scale between them linearly; the conversion goes through a division:
L/100km = 235.21 ÷ MPG(US)
US vs UK gallons
A US gallon (3.785 L) is smaller than a UK/imperial gallon (4.546 L), so the same car reads differently: MPG(UK) figures are about 20% higher than MPG(US) for the identical efficiency. Always check which gallon a figure uses — this tool handles both.
Reference points
| MPG (US) | L/100km |
|---|---|
| 20 | 11.76 |
| 30 | 7.84 |
| 40 | 5.88 |
| 50 | 4.70 |
Where it helps
- Comparing a US-spec car (MPG) to a European one (L/100km)
- Reading a foreign car review in unfamiliar units
- Checking real efficiency against a manufacturer's quoted figure
Worked example
A US car rated at 30 MPG — how efficient is that in European terms? Because the units are inverse, you divide: 235.21 ÷ 30 = 7.84 L/100km. Going the other way, a European car at 5 L/100km is 235.21 ÷ 5 = 47.04 MPG (US). Note the trap in the numbers: improving from 20 to 30 MPG (a 10-MPG gain) saves 11.76 − 7.84 = 3.9 L/100km, but going from 40 to 50 MPG (also 10 MPG) only saves 5.88 − 4.70 = 1.2 L/100km. The same MPG gain saves far less fuel at high efficiency — a real insight the L/100km view reveals that MPG hides.
FAQ
How do I convert MPG to L/100km?+
Divide 235.21 by the US MPG figure: 30 MPG = 235.21 ÷ 30 = 7.84 L/100km. Because the units are inverse, it's a division, not a multiplication. The tool does it for you.
Why are MPG and L/100km inversely related?+
MPG measures distance per unit of fuel (higher is better); L/100km measures fuel per unit of distance (lower is better). One is the reciprocal of the other, so the conversion divides rather than scales.
What is the difference between US and UK MPG?+
A US gallon (3.785 L) is smaller than a UK gallon (4.546 L), so the same car reads about 20% higher in MPG (UK). Always check which gallon a figure uses; the converter supports both.
Is a higher or lower number better?+
Higher is better for MPG and km/L (more distance per fuel); lower is better for L/100km (less fuel per distance). That reversal is exactly why the converter is useful.
What is a good fuel economy figure?+
Roughly, 40+ MPG (US) or under 6 L/100km is efficient for a petrol car; hybrids and small cars do better. Electric vehicles use a different measure (kWh/100km) entirely.
Why does the same MPG gain save less fuel at high efficiency?+
Because of the inverse relationship: at low MPG each extra mile-per-gallon cuts a lot of fuel, but at high MPG the same gain cuts little. The L/100km view makes the real fuel saving visible.