Fuel efficiency units explained (MPG vs L/100km)

Fuel efficiency sounds simple — “how good is this car on gas?” — but the confusion starts because different countries measure “good” in different ways. The United States commonly uses miles per gallon (MPG), while many other places use liters per 100 kilometers (L/100km). Those units don’t just look different — they behave differently too.

MPG is a “higher is better” number. If your car goes 30 miles on one gallon, that’s better than 20 miles on one gallon. So bigger MPG means better fuel economy.

L/100km is a “lower is better” number. It means: “how many liters of fuel does it take to drive 100 kilometers?” If your car uses 7 L to go 100 km, that’s better than using 10 L. So smaller L/100km means better fuel economy.

This is why people sometimes feel like L/100km is “more honest”: the number rises in a straightforward way when you’re burning more fuel. MPG can be psychologically tricky because it’s a reciprocal measurement — improving from 10 MPG to 20 MPG is a huge fuel savings, while improving from 30 MPG to 40 MPG is a smaller fuel savings, even though both are “+10 MPG”.

There’s a second trap: MPG (US) and MPG (UK) are not the same. Both use “miles,” but they use different gallons. A US gallon is about 3.785 liters, while a UK/Imperial gallon is about 4.546 liters. Because the UK gallon is larger, MPG (UK) values look bigger than MPG (US) for the same car. That’s not “better mpg” — it’s a different gallon.

This page converts between:

• MPG (US) — miles per US gallon
• MPG (UK) — miles per Imperial gallon
• L/100km — liters used per 100 km (lower is better)
• km/L — kilometers per liter (higher is better)
• L/100 miles — liters used per 100 miles

Internally, the calculator uses L/100km as the “base” unit because it converts cleanly to trip fuel usage: if you know liters per 100 km, you can multiply by your distance to estimate liters used.

The most popular conversion is MPG ↔ L/100km. These two constants show up all over the internet, and they come from unit math:

• L/100km = 235.214583 ÷ MPG (US)
• L/100km = 282.480936 ÷ MPG (UK)

And the reverse:

• MPG (US) = 235.214583 ÷ (L/100km)
• MPG (UK) = 282.480936 ÷ (L/100km)

For km/L, the relationship is extra intuitive:

• km/L = 100 ÷ (L/100km)
• L/100km = 100 ÷ (km/L)

For liters per 100 miles:

• L/100km = (L/100 miles) × 0.621371 (because 100 miles ≈ 160.934 km)

• 30 MPG (US) → L/100km = 235.214583 ÷ 30 ≈ 7.84
• 7.0 L/100km → MPG (US) = 235.214583 ÷ 7 ≈ 33.60
• 15 km/L → L/100km = 100 ÷ 15 ≈ 6.67

If you enter a trip distance, the calculator estimates fuel used like this:

• Liters used = (Trip distance in km ÷ 100) × (L/100km)

If you also enter fuel price, it converts that price to “per liter” (when needed) and multiplies:

• Trip cost = Liters used × (price per liter)

This isn’t trying to be a perfect “all-in” road trip budget tool — it’s a fast estimate you can screenshot and send to friends: “If we take the SUV, fuel is about $X. If we take the sedan, it’s about $Y.”

Frequently Asked Questions

• Why does my MPG look different in US vs UK?

  Because the “gallon” is different. A UK/Imperial gallon is larger than a US gallon, so MPG (UK) numbers appear higher. You must convert before comparing.

• Is higher MPG always better?

  Yes for fuel economy — but also consider fuel type, maintenance, reliability, and whether the MPG is highway, city, or combined. Real-world driving can differ from sticker numbers.

• Why do some people prefer L/100km?

  It’s easier to connect to fuel cost: liters per 100 km directly tells you how much fuel you burn over a fixed distance. MPG is a reciprocal number, so changes can feel “nonlinear.”

• Can I estimate fuel used for a trip with this?

  Yes — enter distance and you’ll get liters used (and cost if you add fuel price). It’s a simple estimate, not a guarantee.

• What’s the best unit to compare cars globally?

  L/100km is great because it’s widely used and converts neatly to trip fuel usage. km/L is also intuitive for many people.