Fuel Efficiency Converter
Convert between MPG (US and UK), litres per 100km, km per litre, miles per litre, and more. Instantly see all fuel economy values side by side.
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Why doubling your MPG doesn’t halve your fuel bill, and how this converter gets the math right
MPG and litres per 100 km look like they should convert with simple multiplication, the way metres convert to feet. They don’t. One measures distance per unit of fuel, the other measures fuel per unit of distance, and those two quantities are inverses of each other, not linear multiples. Going from 20 MPG to 40 MPG saves far more fuel over a fixed distance than going from 40 MPG to 60 MPG, even though both jumps look like 20 more miles per gallon on paper. This tool is built around that inverse relationship instead of pretending it isn’t there.
Everything computes in your browser with ordinary division and multiplication. No trip data, location or vehicle information is transmitted anywhere.
Why the internal reference unit is L/100km, not MPG
The tool picks litres per 100 kilometres as its hub unit because it is the one format on this list that scales linearly: double the L/100km figure and you’ve genuinely doubled the fuel burned over that distance. MPG style units do not have that property, so they get special inverse handling rather than a flat factor.
vd, volume per distance, which scales directly against L/100km, or dv, distance per volume, which needs an inverse calculation.
100 / (val × 1.609344 / 3.785411784), folding the mile to kilometre ratio and the US gallon to litre ratio into one inverted expression.
(100 / l100) × 3.785411784 / 1.609344, undoing the inversion in the opposite order.
L/100km × distance / 100, multiplied by a price per litre you supply, giving a trip fuel cost alongside the raw conversion.
Full unit table, and how each relates to the L/100km hub
| Unit | Symbol | Type | Relationship to L/100km |
|---|---|---|---|
| Litres per 100 km | L/100km | Volume/distance | 1 (base unit) |
| MPG (US) | MPG | Distance/volume | Inverse, gallon = 3.785411784 L |
| MPG (Imperial/UK) | MPG(UK) | Distance/volume | Inverse, gallon = 4.54609 L |
| Kilometres per litre | km/L | Distance/volume | Inverse, 100/L100km |
| Miles per litre | mi/L | Distance/volume | Inverse, ÷1.609344 |
| Litres per 10 km | L/10km | Volume/distance | × 10 (linear scale) |
| Litres per km | L/km | Volume/distance | × 100 (linear scale) |
| US gallons per 100 miles | gal/100mi | Volume/distance | × 1/0.425144 (linear scale) |
Two things worth understanding about the numbers
The diminishing returns curve
Because MPG is an inverse quantity, equal MPG increases save less and less fuel as the baseline improves. A jump from 10 to 20 MPG cuts fuel use per mile in half. A jump from 40 to 50 MPG only cuts it by a fifth. L/100km avoids this distortion entirely, which is why most of the world outside the US and UK standardized on it.
Built-in trip cost estimator
Once a L/100km figure is cached from your conversion, entering a trip distance and a fuel price per litre multiplies them together, litres used times price, to give a real cost estimate in whatever currency symbol you select.
Where these standards come from
MPG figures on new US vehicles come from EPA dynamometer testing under standardized city and highway cycles. The Imperial gallon was formally defined in the UK Weights and Measures Act of 1824 as the volume of 10 pounds of water, a definition that survived long after most of the Commonwealth switched to metric for everything else. L/100km, by contrast, is the natural output of European and international test cycles like the WLTP, and it scales the way most people intuitively expect fuel consumption to behave: more litres for more kilometres, in a straight line.
Fuel economy references
- US Department of Energy, fueleconomy.gov documents EPA testing methodology behind US MPG ratings.
- UK Weights and Measures Act 1824 is the historical source defining the Imperial gallon still used in UK MPG figures.
- UNECE, Worldwide Harmonized Light Vehicle Test Procedure covers the WLTP cycle behind most L/100km fuel economy labels sold today.
Comparing economy across markets
Comparing a US car’s MPG rating against a European model’s L/100km sticker before an international purchase, working out real road trip fuel costs once you know your car’s consumption and current fuel prices, understanding why an upgrade from a 15 MPG truck saves more money than the same MPG jump on an already efficient sedan, converting a UK MPG figure to the US equivalent before comparing reviews across regions, and checking fleet fuel budgets where consumption is tracked in litres per 100 km but reported to management in miles per gallon.
Common Questions
FAQ: Fuel Efficiency Converter
For US MPG: L/100km = 235.214 ÷ MPG. For UK MPG: L/100km = 282.481 ÷ MPG. The formula works because MPG and L/100km are inversely related — one is distance per volume, the other is volume per distance.
A UK (imperial) gallon equals 4.546 litres, while a US gallon is only 3.785 litres. Since a car travels the same distance on more liquid, UK MPG figures are always about 20% higher than US MPG for the same vehicle.
For petrol/diesel cars: under 6 L/100km (above 47 MPG US) is excellent; 6-8 L/100km is average; above 12 L/100km is poor. Hybrid and EV equivalents are expressed differently. Driving style and conditions affect real-world figures significantly.
Both measure the same thing. A higher MPG means your vehicle travels farther using the same amount of fuel, while a lower L/100km means it consumes less fuel over a fixed distance.
Yes. Multiply US MPG by approximately 1.201 to get UK MPG. To convert UK MPG to US MPG, divide by 1.201.
The United States commonly uses miles per gallon (US MPG), the UK uses imperial MPG, while most European and many other countries use litres per 100 kilometres (L/100km).
Yes. Fuel economy units are independent of fuel type. The conversion works for petrol, diesel, hybrid, and any vehicle whose efficiency is measured in MPG or L/100km.
Driving speed, acceleration, traffic, tyre pressure, weather, road conditions, vehicle maintenance, cargo weight, and air conditioning use all influence actual fuel consumption.
L/100km is easier to compare because lower values always indicate better efficiency. Many countries adopted it as the standard metric fuel consumption measurement.
Yes. It uses the internationally accepted conversion constants for US gallons, UK gallons, litres, miles, and kilometres, providing precise and reliable results.
From the blog
Reading around units
Where measurement systems came from and why two of them still argue.
