Litres to gallons
One litre is 0.264172 US gallons, because a US liquid gallon is defined as exactly 231 cubic inches, which works out at 3.785411784 litres. The imperial gallon used in Britain is a different unit at exactly 4.54609 litres, so the same volume is only 0.219969 imperial gallons. Whenever a gallon appears without a nationality attached, find out which one is meant before you use the number.
How to convert litres to gallons
Two gallons, and a pint that runs the other way
The US gallon is about seventeen per cent smaller than the imperial gallon. That much is fairly well known. What catches people is that the smaller gallon is divided into larger fluid ounces: a US fluid ounce is 29.5735 ml and an imperial fluid ounce is 28.4131, so the imperial one is the smaller measure of the two.
The reconciliation is in the count. A US pint is sixteen US fluid ounces and an imperial pint is twenty imperial ones, which puts the imperial pint at 568.26 ml against the American 473.18. So a British pint of beer is a fifth larger than an American one even though a British fluid ounce is smaller. Recipes are where this bites hardest, because a "cup" is 236.6 ml in America, 250 ml if the book is metric, and roughly 284 ml if somebody has taken half an imperial pint.
Fuel economy does not convert the way it looks
Miles per gallon carries the gallon inside it, so a British mpg figure and an American one are not the same kind of number. Thirty miles per US gallon is 36.03 miles per imperial gallon. A car advertised at 40 mpg in Britain is doing 33.3 by American reckoning, and a model sold on both sides of the Atlantic will honestly carry both figures on its own brochures.
Litres per 100 km is the third convention and it runs backwards, since a lower number is better. Being a reciprocal, it also explains why mpg figures cannot be averaged. Drive 100 miles at 40 mpg and another 100 at 10 mpg and you have burned 2.5 plus 10 gallons to cover 200 miles, which is 16 mpg, not the 25 an average would suggest. The conversion is 235.215 divided by the US mpg figure, so 30 US mpg is 7.84 L/100 km. For imperial mpg the constant is 282.481.
Where each gallon came from
The US gallon is the old English wine gallon, fixed at 231 cubic inches by a statute of 1707 and kept by the United States after Britain moved on. Britain replaced its several competing gallons with a single imperial gallon in 1824, originally defined as the volume of ten pounds of distilled water under stated conditions, and pinned it at exactly 4.54609 litres in 1985. Two systems, each internally consistent, mutually incompatible for two centuries.
What people use it for
- Reading a car’s fuel tank capacity in gallons
- Comparing a British mpg figure against an American one
- Sizing an aquarium or water butt listed in litres
- Costing fuel priced per litre against a price per gallon
- Converting a drum or IBC volume for a US order
- Scaling an American recipe that measures in cups and pints
Questions
The US liquid gallon, 3.785411784 litres. It is the one meant whenever a gallon is quoted in the United States.
Exactly 4.54609 litres, about twenty per cent more than a US gallon. It was originally the volume of ten pounds of water.
Because the two systems divide their gallons differently. A US gallon is 128 US fluid ounces and an imperial gallon is 160 imperial ones, so the imperial ounce ends up smaller even though the imperial gallon is bigger.
The imperial one, at 568.26 ml against the American 473.18. That is twenty imperial fluid ounces against sixteen US ones.
A separate unit of 4.40488 litres, one eighth of a Winchester bushel, used for grain and produce. It never applies to liquids.
Multiply US mpg by 1.20095 to get imperial mpg, or divide the other way. Thirty US mpg is 36.03 imperial mpg for exactly the same car.
Divide 235.215 by the US mpg figure, or 282.481 by the imperial one. Thirty US mpg is 7.84 L/100 km.
Because mpg is a reciprocal: it is distance over fuel, not fuel over distance. Averaging it weights the efficient leg far too heavily. Convert to litres per 100 km, average that, and convert back.
A petroleum barrel is 42 US gallons, so 158.99 litres. Other barrels are different sizes entirely: a US beer barrel is 31 US gallons and a British brewer’s barrel is 36 imperial gallons.
Close but not equal. A US quart is 0.9464 litres, so a litre is about 5.7 per cent more. There are four quarts to a gallon in both systems.