Convert mpg (US & UK), L/100km, km/L and mi/L all at once — with a chart showing why L/100km is the inverse of mpg.
mpg (US)
30
L/100km
7.84
Calculator
Converted
7.84 L/100km
mpg (UK)
36.03
km/L
12.75
mi/L
7.93
Miles per gallon (US)
30 mpg
Miles per gallon (UK/Imperial)
36.03 mpg
Litres per 100 km
7.84 L/100km
Kilometres per litre
12.75 km/L
Miles per litre
7.93 mi/L
Litres to drive 100 km — lower is more efficient
How fuel economy conversion works
Miles per gallon and litres per 100 km are not simply proportional — they are inversely related. Doubling your mpg does not halve your fuel cost per kilometre in a linear sense; the curve is hyperbolic. That is why moving from 8 L/100km to 6 L/100km saves more fuel than moving from 6 to 4 L/100km at the same annual distance.
The calculator routes every conversion through L/100km as the pivot using exact NIST constants: 1 US gallon = 3.785 411 784 L; 1 Imperial gallon = 4.546 09 L; 1 mile = 1.609 344 km. US mpg and UK mpg differ because a UK (Imperial) gallon is about 20 % larger than a US gallon.
Why is UK mpg higher than US mpg for the same car?
A UK (Imperial) gallon is 4.546 09 L, about 20 % larger than a US gallon (3.785 412 L). So the same car travelling the same distance uses fewer Imperial gallons than US gallons, giving a higher mpg figure in the UK.
Why does the L/100km scale feel counter-intuitive?
Lower L/100km means better efficiency, the opposite of mpg where higher is better. This inversion trips people up when comparing cars from different markets. The comparison chart here makes the relationship visual — a bar that barely reaches the left edge means your car sips fuel.
How do I convert L/100km to mpg quickly in my head?
For US mpg, divide 235 by the L/100km figure. For UK mpg, divide 282 by the L/100km figure. These are the approximate pivot constants derived from the exact NIST conversion factors.
Results are estimates. Verify with a professional for important decisions.
About this calculator
This converter translates a fuel-economy figure between the five units used around the world: mpg (US gallon), mpg (Imperial/UK gallon), litres per 100 kilometres, kilometres per litre, and miles per litre. Use it to compare a European car spec sheet with a North American one, check whether your vehicle meets a fuel-economy standard stated in unfamiliar units, or simply satisfy curiosity about what your fuel figure means in another system.
How to read your results
Type a value in any unit and every other field updates instantly. The key insight is that L/100km is an inverse efficiency measure — a lower number is better (you burn fewer litres), while mpg and km/L are direct measures where a higher number is better. The visual comparison bar makes that inversion visible at a glance. US and Imperial mpg differ because the US gallon (3.785 L) is smaller than the Imperial gallon (4.546 L), so the same car always shows a higher mpg number on the Imperial scale.
Worked example
Enter 30 in the mpg (US) field.
The converter returns 7.84 L/100km, 36.03 mpg (UK), 12.75 km/L, and 7.93 mi/L. Notice that 30 US mpg and 36 UK mpg describe the exact same fuel efficiency — the difference is entirely due to the gallon size.
Frequently asked questions
Why is US mpg always lower than UK mpg for the same car?
The US gallon holds 3.785 litres while the Imperial gallon holds 4.546 litres — about 20% more. Because the Imperial gallon is larger, a car travels more miles per Imperial gallon than per US gallon, so UK mpg figures are always about 20% higher for the same vehicle. Neither number is 'wrong'; they simply use different gallon definitions.
Why does a lower L/100km mean better efficiency?
L/100km measures fuel consumed per fixed distance — it asks 'how many litres does it take to cover 100 km?' Less fuel for the same distance means better efficiency, so lower is better. mpg and km/L ask the opposite question — 'how far can I go on one unit of fuel?' — so higher is better for those. Because L/100km is the reciprocal of km/L, doubling your km/L halves your L/100km.
Which unit should I use?
Use L/100km if you are in continental Europe, Australia, or comparing against EU regulatory figures. Use mpg (US) for North American specs, and mpg (UK) for British vehicles or insurance documents from the United Kingdom. km/L is common in Japan, India, and South-East Asia. mi/L is rare but occasionally appears in older UK engineering references.
Does the converter account for actual driving conditions?
No — this is a pure unit conversion. It accurately translates a given efficiency figure from one unit to another, but it does not model real-world variables such as driving style, speed, load, tyre pressure, temperature, or fuel grade. Always compare figures measured under the same test cycle (for example, WLTP vs EPA) before drawing conclusions.
What are the exact conversion constants used?
The calculator uses NIST SP-811 exact definitions: 1 US gallon = 3.785 411 784 L, 1 Imperial gallon = 4.546 09 L, 1 mile = 1.609 344 km. These give pivot constants of 235.215 (L·100km⁻¹ × mpgUS) and 282.481 (L·100km⁻¹ × mpgUK). All intermediate rounding is deferred until final display.
How it's calculated
Every input is first converted to L/100km as an internal pivot, then fanned out to the remaining four units. For mpg (US): L/100km = 235.215 ÷ mpg(US); for mpg (UK): L/100km = 282.481 ÷ mpg(UK); for km/L: L/100km = 100 ÷ km/L; for mi/L: convert to km/L by multiplying by 1.609 344, then invert as above. From L/100km the other units follow: km/L = 100 ÷ L/100km, mpg(US) = 235.215 ÷ L/100km, mpg(UK) = 282.481 ÷ L/100km, mi/L = km/L ÷ 1.609 344. L/100km is chosen as the pivot precisely because its inverse relationship with mpg is the main conceptual challenge this calculator is designed to clarify. The US gallon is smaller than the Imperial gallon, so the same car always shows a higher mpg on the Imperial scale; the converter makes this difference explicit by computing both simultaneously.
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