The United Kingdom and the United States both measure liquids in gallons, pints, and fluid ounces, but they do not mean the same sizes. The Imperial gallon holds 4.54609 litres and the US gallon holds 3.78541 litres, so the Imperial gallon is about twenty percent bigger. The smaller units split too, and in the opposite direction in one case: the US fluid ounce is about four percent larger than the Imperial fluid ounce, even though the US gallon is twenty percent smaller, because the two gallons are divided into different numbers of ounces. This guide lays the two systems out side by side — gallons, quarts, pints, cups, and fluid ounces — explains why they split, and shows how to convert fuel economy between them without the usual twenty-percent error.

The two gallons side by side

Both systems divide the gallon into quarts and pints, and both divide the pint into fluid ounces — but the counts differ. The Imperial gallon is divided into 4 quarts, 8 pints, and 160 fluid ounces. The US gallon is divided into 4 quarts, 8 pints, and 128 fluid ounces. Same structure, different ounce count, and that is why the fluid ounce flips: the smaller US gallon divided into fewer ounces gives a larger ounce.

Table notes: Imperial gallon = 4.54609 litres exactly; US gallon = 3.785411784 litres exactly. Pints and quarts follow from the gallon; the US cup is 8 US fluid ounces.

Two rows in the table deserve attention. The Imperial pint is 568.26 ml against the US pint at 473.18 ml — a full 95 ml more, about the volume of a small glass of water. And the fluid ounce row runs the other way: the US fluid ounce is the larger one. A recipe or a medicine measure that swaps the fluid ounces is wrong by about four percent, which is small but real, and a recipe that swaps the pints is wrong by twenty percent, which is not.

Why they split: a short history

Before 1824, Britain used several gallons at once: a wine gallon, an ale gallon, and a corn gallon, each with its own size, and the confusion was a standing complaint of merchants. The Weights and Measures Act of 1824 swept them away and defined a single Imperial gallon as the volume of ten pounds of distilled water at 62 degrees Fahrenheit. The United States, independent by then, kept the wine gallon it had inherited: 231 cubic inches, fixed by Queen Anne in 1707, which became the US gallon.

Neither side has since changed its gallon. The United States still sells fuel and milk by the US gallon. The United Kingdom sells fuel by the litre but still quotes older fuel-economy figures in miles per Imperial gallon, and the pint of beer is still an Imperial pint. The split is a historical accident that survives because both systems are embedded in daily life — in road signs, in recipes, and in the way people talk about a car’s thirst.

Pints and fluid ounces

The pint is where the difference is most often felt. An Imperial pint of beer or milk is 568.26 ml; a US pint is 473.18 ml. Order a pint in London and you get a fifth more than a pint in New York, and the glass is sized to match. The same applies in the kitchen: a recipe calling for two pints means 1.14 litres in Britain and 0.95 litres in the United States, a difference of nearly 200 ml across the recipe.

The fluid ounce is the one unit that runs against the pattern. A US fluid ounce is 29.5735 ml, because it is one 128th of a US gallon of 3.785411784 litres. An Imperial fluid ounce is 28.4131 ml, because it is one 160th of an Imperial gallon of 4.54609 litres. The US fluid ounce is about four percent larger, even though the US gallon is twenty percent smaller, because the two gallons are divided into different numbers of ounces. It is the single most counter-intuitive fact in the two systems, and the one most worth remembering when a measure calls for fluid ounces without saying which system it means.

Miles per gallon in two systems

Fuel economy is where the two gallons cause the most confusion, because a miles-per-gallon figure without a gallon is meaningless. A car that returns 40 miles per Imperial gallon returns 33.3 miles per US gallon for the same fuel use — the same car, the same road, a number a fifth lower, because the US gallon is a fifth smaller. In litres per 100 km, that same 40 Imperial mpg is 7.06 L/100 km, and 30 US mpg is 7.84 L/100 km.

The conversion rules are simple. To turn Imperial mpg into US mpg, multiply by 0.83267 (the ratio of the gallons, 3.78541 ÷ 4.54609). To turn US mpg into Imperial mpg, multiply by 1.20095. To turn either into litres per 100 km, divide 282.48094 by Imperial mpg, or 235.21458 by US mpg. The companion guide on how many litres are in a gallon gives the exact gallon figures these ratios come from.

Table notes: US mpg = Imperial mpg × 0.83267; litres per 100 km = 282.48094 ÷ Imperial mpg.

Which gallon does each country use?

The United States uses the US gallon for fuel, milk, and paint, and the US pint and fluid ounce throughout its recipes and measures. The United Kingdom uses litres at the fuel pump and on most packaging, but the Imperial pint survives for draught beer and milk, and miles per Imperial gallon is still the figure older drivers quote for a car’s economy. Canada sells fuel by the litre, but Imperial-gallon figures still appear in conversation and in older consumption figures, because Canada used Imperial gallons before metrication — a Canadian miles-per-gallon figure is usually Imperial, and should be checked before it is compared with an American one.

Elsewhere, the litre is the standard, and the gallon appears only in trade with the United States or the United Kingdom. Liberia and Myanmar are the other countries that use US gallons in daily life. The practical rule is the same everywhere: a gallon figure needs its country attached before it can be used, and a litre figure does not.

Converting between the systems

Convert in two steps: gallons to litres first, then litres to the other gallon. To turn Imperial gallons into US gallons, multiply by 1.20095 — so 10 Imperial gallons is 12.01 US gallons. To turn US gallons into Imperial gallons, multiply by 0.83267 — so 10 US gallons is 8.33 Imperial gallons. For pints, the same ratios apply: an Imperial pint is 1.20095 US pints, and a US pint is 0.83267 Imperial pints.

For fluid ounces, the ratio flips: a US fluid ounce is 1.04084 Imperial fluid ounces, and an Imperial fluid ounce is 0.96076 US fluid ounces. The flip is the signature of the two systems — the gallon, quart, and pint are all bigger on the Imperial side, and the fluid ounce alone is bigger on the US side — and it is the detail that catches people who assume the whole Imperial system is uniformly larger.

Sources: Imperial gallon defined as 4.54609 litres exactly (UK Weights and Measures Act 1985); US gallon defined as 231 cubic inches (3.785411784 litres exactly). Imperial gallon = 160 Imperial fluid ounces; US gallon = 128 US fluid ounces. Fuel-economy conversions follow from the gallon definitions.