This converter changes fuel economy between miles per gallon (MPG) and litres per 100 km (L/100km), in both directions, with a choice of UK or US gallons. The two measures run in opposite senses — higher MPG is better, lower L/100km is better — so the tool also helps compare figures across regions and avoid the common mistake of comparing a UK figure to a US one directly.
Why UK and US gallons produce different numbers
A UK (imperial) gallon is 4.54609 litres, while a US gallon is 3.785412 litres — about 20% smaller. The practical consequence: the same physical fuel economy expressed in UK MPG produces a higher number than in US MPG. For example, a car that does 50 UK MPG only does about 41.6 US MPG. When comparing a European spec sheet (which typically shows L/100km) to a US window sticker (US MPG), this tool handles all the conversions in one step.
How it works
The conversion combines three exact unit factors: 1 mile = 1.609344 km, 1 UK gallon = 4.54609 L, and 1 US gallon = 3.785411784 L. With the chosen gallon size in litres (G):
- MPG to L/100km:
L/100km = (G × 100) ÷ (MPG × 1.609344) - L/100km to MPG:
MPG = (G × 100) ÷ (L/100km × 1.609344)
The two quantities are reciprocal once scaled by the unit factors, which is why an efficient car has high MPG but low L/100km.
Reference conversion table
| MPG (US) | MPG (UK) | L/100km |
|---|---|---|
| 25 | 30.0 | 9.41 |
| 30 | 36.0 | 7.84 |
| 40 | 48.0 | 5.88 |
| 50 | 60.0 | 4.70 |
| 60 | 72.0 | 3.92 |
Practical situations where this matters
Buying an imported vehicle. A Japanese domestic market car or a European grey import will have its fuel economy stated in L/100km. Use this converter to check whether the real-world consumption matches what you expect in MPG terms before purchase.
Comparing insurance or lease quotes across regions. Insurers and some fleet managers in continental Europe quote in L/100km; UK and US counterparts quote in MPG. A mismatch in gallon definition inflates the apparent difference further.
Calculating fuel costs on a road trip. If you know your car’s L/100km figure and the local fuel price per litre, you can convert to equivalent MPG to estimate how many miles per tank you will actually get.
Calculating fuel cost from L/100km
Once you have the L/100km figure, calculating fuel cost for a trip is straightforward:
fuel needed (L) = (distance_km / 100) × L/100km
trip cost = fuel needed × price_per_litreFor example, a 500 km trip in a car at 6.0 L/100km needs (500/100) × 6 = 30 litres. At £1.50/L that is £45 in fuel.
EV equivalent: Wh/km and kWh/100km
Electric vehicles use an analogous measure: kWh per 100 km (or Wh/km). A lower figure means more efficient. The conversion is the same in structure — energy per unit distance — so the habit of thinking in L/100km transfers directly to EV range planning. If you are comparing an ICE car at 5.5 L/100km to an EV at 18 kWh/100km, the energy-cost comparison needs only the local fuel price per litre and electricity tariff per kWh.
The conversion runs entirely in your browser, so nothing is uploaded.