The One Number Behind Every MPG to L/100km Conversion: 235.215
Our MPG to L/100km converter runs on a single magic number: 235.215. Divide it by any US MPG figure and you get L/100km; divide it by any L/100km figure and you get US MPG back. A car rated at 30 US MPG burns 235.215 ÷ 30 = 7.84 L/100km. A European spec sheet that says 6 L/100km is 235.215 ÷ 6 = 39.2 US MPG. That's the whole trick — but there are two traps that catch people every time: the US gallon and the UK gallon aren't the same size, and the relationship between MPG and L/100km is not a straight line. This page handles both, and below we'll show you exactly why they matter.
US Gallon vs UK Gallon: Why Your Conversion Can Be 20% Off
Here's the mistake that wrecks more conversions than any other: assuming a gallon is a gallon. A US gallon is 3.785 liters. A UK (imperial) gallon is 4.546 liters— about 20% larger. Because a UK gallon holds more fuel, a car will always post a higher MPG number on the UK scale than the US scale, even though the actual efficiency is identical.
Take a car that does 8.0 L/100km. In US MPG that's 29.4. In UK MPG it's 35.3. Same car, same fuel burn, two different numbers. This is why a British review raving about "52 mpg" sounds far better than the US window sticker — it's 52 UK MPG, which is only about 43 US MPG. Always confirm which gallon a figure uses before you compare cars across the Atlantic.
| L/100km | US MPG | UK MPG | km/L | Real-world example |
|---|---|---|---|---|
| 12.0 | 19.6 | 23.5 | 8.3 | Full-size SUV / pickup |
| 9.0 | 26.1 | 31.4 | 11.1 | Midsize crossover |
| 7.8 | 30.2 | 36.2 | 12.8 | Compact sedan |
| 5.5 | 42.8 | 51.4 | 18.2 | Efficient hatchback |
| 4.5 | 52.3 | 62.8 | 22.2 | Hybrid |
UK MPG runs roughly 20% higher than US MPG for the same car because the imperial gallon is larger.
The Conversion Formulas (and the Constants Behind Them)
Every conversion on this page comes from three exact constants: 1 US gallon = 3.785411784 liters, 1 UK gallon = 4.54609 liters, and 1 mile = 1.609344 km. Combine them and you get the working formulas:
US MPG → L/100km: L/100km = 235.215 ÷ MPG
UK MPG → L/100km: L/100km = 282.481 ÷ MPG
L/100km → km/L: km/L = 100 ÷ (L/100km)
US MPG → UK MPG: UK MPG = US MPG × 1.20095
Where does 235.215 come from? One US MPG equals 1.609344 km per 3.785412 liters, which is 0.42514 km per liter. To turn "km per liter" into "liters per 100 km" you flip it and scale by 100: 100 ÷ 0.42514 = 235.215. The UK constant (282.481) is bigger purely because the imperial gallon is larger. If you want to measure your own MPG first from a fill-up before converting it, our MPG calculator does the miles-divided-by-gallons math and shows the L/100km equivalent alongside it.
Why L/100km and MPG Don't Move in a Straight Line
This is the single biggest misunderstanding about fuel economy units, and it costs buyers real money. MPG measures distance per fuel (more is better). L/100km measures fuel per distance (less is better). They're inverses, not opposites — which means equal MPG steps are not equal L/100km steps.
Going from 10 to 20 MPG cuts consumption from 23.5 to 11.8 L/100km — a huge 11.7-liter drop. Going from 40 to 50 MPG — the same 10 MPG gain — only drops consumption from 5.9 to 4.7 L/100km, barely 1.2 liters. That's why replacing a thirsty 15 MPG truck with a 20 MPG one saves far more fuel than upgrading a 40 MPG car to 50 MPG, even though the second jump looks bigger on the MPG scale. Europe uses L/100km precisely because it reflects actual fuel burned, and it makes these savings obvious.
| MPG step (+10) | L/100km before | L/100km after | Fuel saved per 100 km |
|---|---|---|---|
| 15 → 25 MPG | 15.7 | 9.4 | 6.3 L |
| 25 → 35 MPG | 9.4 | 6.7 | 2.7 L |
| 35 → 45 MPG | 6.7 | 5.2 | 1.5 L |
Same 10 MPG improvement, wildly different real fuel savings. Figures use US MPG.
When the Converter Alone Isn't Enough
A unit conversion tells you the rating, not the cost. If you've converted a European 6 L/100km spec to 39 US MPG and want to know what that means at the pump, you need fuel price and mileage, not just the number. Plug the converted MPG into our gas mileage calculator to turn efficiency into dollars per month, or use the fuel savings calculator to compare two cars side by side once both are in the same unit. The converter is the first step; the cost tools finish the job.
One more caution: don't convert km/L and L/100km as if they were the same kind of number. km/L is an efficiency figure (higher is better) and L/100km is a consumption figure (lower is better). A car at 12.8 km/L is a strong 7.8 L/100km — you flip and scale by 100, you don't just relabel the digits. For the official US testing methodology behind window-sticker MPG, see the EPA test schedules at fueleconomy.gov, and the NIST Office of Weights and Measures documents the exact gallon and mile definitions used above.