What this converter works out
This converts fuel consumption between the four scales in common use: miles per gallon on the UK measure, miles per gallon on the US measure, litres per 100 kilometres, and kilometres per litre. It also gives a cost per mile at the fuel price you enter.
Two traps account for most confusion. The UK and US gallon are different sizes, so the same car scores about 20% higher on the UK scale. And miles per gallon and litres per 100 km run in opposite directions: a higher mpg is better, a lower L/100 km is better.
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Before you calculate
- The UK gallon and the US gallon are different. A UK gallon is 4.546 litres; a US gallon is 3.785 litres. The same car scores about 20% higher in UK mpg than in US mpg, which is why figures quoted on American websites look flattering.
- Miles per gallon measures distance per unit of fuel, so higher is better. Litres per 100 km measures fuel per unit of distance, so lower is better. The two run in opposite directions.
- Manufacturer figures come from a standardised laboratory test. Real driving almost always returns less.
Because one is the reciprocal of the other, equal steps mean different things. Improving from 20 to 25 mpg saves far more fuel over the same distance than improving from 50 to 55 mpg, even though both are a five-unit gain. Litres per 100 km makes this obvious, which is why most of Europe uses it.
How this calculator works
1 UK gallon = 4.54609 litres
1 US gallon = 3.785412 litres
1 mile = 1.609344 km
L/100 km = (100 × litres per gallon) ÷ (mpg × km per mile)Everything is converted to a common basis of litres per 100 kilometres, then back out to whichever units you asked for. The cost per mile multiplies the litres used per mile by your fuel price.
Worked example: 45 mpg
A car returning 45 miles per gallon on the UK scale, with fuel at 140p a litre:
- Litres per 100 km: (100 × 4.54609) ÷ (45 × 1.609344) = 6.28 L/100 km
- US equivalent: 37.5 mpg
- Kilometres per litre: 15.9 km/L
- Cost per mile: 14.1p, so a 100-mile journey costs about £14
Common mistakes
- Comparing UK and US mpg figures directly. A US figure will always look worse for the same car.
- Assuming the official figure is achievable. Test cycles are standardised laboratory procedures, not typical journeys.
- Working out mpg from a brim-to-brim fill without a full tank at both ends. Fill to the same click-off point twice, or the result is meaningless.
- Treating a single tank as representative. Weather, load, tyre pressures and the mix of roads all move the figure. Average several tanks.
- Comparing petrol and diesel on mpg alone. Diesel is usually priced differently per litre, so cost per mile is the fair comparison.
Frequently asked questions
Why is my real mpg lower than the manufacturer's figure?
Official figures come from a standardised laboratory test designed for comparability between cars, not for predicting your journeys. Short trips from cold, motorway speeds, roof boxes, cold weather, low tyre pressures and a heavy right foot all reduce it. A shortfall of 10 to 20% against the official figure is unremarkable.
Is a UK gallon the same as a US gallon?
No. A UK gallon is 4.546 litres and a US gallon is 3.785 litres, so a UK gallon is about 20% larger. A car doing 45 mpg on the UK scale does about 37.5 mpg on the US scale. Always check which one a figure refers to before comparing.
Which is better, miles per gallon or litres per 100 km?
Litres per 100 km is the more useful measure, because fuel used is proportional to it. Miles per gallon is a reciprocal scale, so the same numerical improvement means very different things at different points on it. Going from 20 to 25 mpg saves more fuel per mile than going from 50 to 55.
How do I measure my own fuel consumption accurately?
Fill the tank to the first click-off, reset the trip meter, drive normally for a few hundred miles, then fill to the first click-off again at the same pump type. Divide the miles by the litres, then multiply by 4.546 for UK mpg. Repeat over several tanks and take the average.
Does the calculator work for electric vehicles?
No. Electric consumption is measured in miles per kilowatt-hour or kilowatt-hours per 100 km, and electricity is priced per kWh rather than per litre. The EV charging cost calculator handles that case.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
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References
These organisations publish the guidance and figures behind this page.
- Vehicle Certification Agency — official UK fuel consumption and emissions figures for new cars
- GOV.UK — vehicle tax bands, emissions rules and approved mileage rates
Sources are checked at publication and can change — how I choose and check references.