Fuel Consumption Converter
Convert km/l, miles per gallon (US & UK) and litres per 100 km.
Note: for L/100km, a lower number means better fuel efficiency — it is an inverse measurement compared to the other units here.
Km/L, mpg, and the units that confuse every road trip abroad
Fuel efficiency might be the most internationally inconsistent unit family drivers regularly deal with. The US and UK both use "miles per gallon," but their gallons are different sizes, so the same car gets a different mpg figure depending on which country's gallon is used. Most of the rest of the world uses kilometres per litre or the inverted metric, litres per 100 kilometres. This tool converts between all of these, including handling the tricky detail that fuel efficiency units can be either "distance per unit fuel" or "fuel per unit distance" — which are inverses of each other, not simple multiples.
US mpg versus UK mpg
A US gallon is 3.78541 litres, while a UK (imperial) gallon is 4.54609 litres — about 20% larger. This means a car quoted at "40 mpg" in a UK car magazine is not the same efficiency as a car quoted at "40 mpg" in a US review; the UK figure represents meaningfully better real-world fuel economy because it's describing more distance per (larger) gallon. Whenever you're comparing fuel economy figures across US and UK sources, converting both into a common unit like km/L or L/100km removes this ambiguity entirely.
Why litres per 100km runs backwards
Litres per 100 kilometres (L/100km) is an inverse-style measurement: unlike miles-per-gallon or km-per-litre, a lower number means better fuel efficiency, because it's describing how much fuel is consumed over a fixed distance rather than how much distance is covered per unit of fuel. This inversion is exactly why fuel-efficiency conversion can't simply reuse the same multiply-by-a-factor method used for length or weight — converting into or out of L/100km requires taking a reciprocal, not just multiplying by a ratio, and this tool handles that calculation for you automatically.
Where this comes up
Car buyers comparing fuel economy figures between US, UK, and European or Asian markets that use different default units, road trip planners estimating fuel costs and how many stops a journey will require, and automotive journalists and enthusiasts converting manufacturer-quoted figures for an international audience all rely on accurate fuel consumption conversion. Because efficiency figures directly affect a trip's fuel budget, getting the US-versus-UK gallon distinction right matters more here than in most other unit categories on this site.
Practical tips
When comparing vehicles from different countries, convert every figure into the same unit — km/L is a good neutral choice — before making a decision, rather than trying to compare "mpg" numbers directly, since you can't tell at a glance whether a source means US or UK gallons. Real-world fuel economy also varies with driving style, terrain, and vehicle load, so treat any converted manufacturer figure as a reference point rather than an exact prediction for your specific trip.
Budgeting a road trip
Once you know a vehicle's efficiency in a consistent unit, estimating a trip's fuel cost is straightforward: divide the total distance by the efficiency figure to get fuel volume needed, then multiply by the local fuel price — this converter handles the unit-matching step so the rest of the calculation stays simple.
Frequently asked questions
Is US mpg the same as UK mpg?
No. A US gallon (3.78541 L) is smaller than a UK imperial gallon (4.54609 L), so a car quoted at the same mpg number in both countries actually has different real-world fuel efficiency — the UK figure represents better economy.
How do I convert mpg to litres per 100km?
Because L/100km is an inverse measurement (lower is better), the conversion involves a reciprocal calculation rather than a simple multiplication. This tool performs that calculation automatically — just enter your mpg value and select L/100km as the output.
What is a good L/100km figure?
Smaller numbers mean better fuel efficiency. A typical modern petrol car might average 6–8 L/100km, while an efficient hybrid can achieve 4 L/100km or lower — figures vary significantly by vehicle type and driving conditions.
How many km/L is 40 mpg?
40 US mpg is approximately 17 km/L. 40 UK mpg is approximately 14.2 km/L, since UK and US gallons differ in size — select the correct mpg standard in the tool for an accurate result.
Why does fuel consumption need a different calculation method than other converters?
Most units in this site scale directly (multiply by a factor), but L/100km measures fuel used per fixed distance rather than distance covered per unit of fuel — the opposite direction — so converting it requires a reciprocal step rather than a straight multiplication.