Conversion from US Liquid pints to Gallons

Information about the US Liquid pint (pt)

The United States liquid pint is a unit of volume equal to exactly 473.176473 millilitres. Its symbol is pt. It is sixteen United States fluid ounces and an eighth of a United States gallon. It is noticeably smaller than the imperial pint of 568 millilitres, because the American pint holds sixteen fluid ounces where the British one holds twenty.

That difference of four ounces is the whole story of the two pints. Both systems descend from the same medieval English measures, but Britain rebuilt its liquid chain in 1824 around a gallon of ten pounds of water, while the United States kept the older wine gallon of 231 cubic inches. The pint inherited the difference, and a British pint is now about a fifth larger than an American one.

The rhyme survives because the American pint fits it. A pint's a pound the world around works for the United States pint, since sixteen fluid ounces of water weigh 1.04 pounds, and it fails in Britain, where a pint of water weighs a pound and a quarter. The saying is a useful reminder that measurement rhymes travel worse than the measurements themselves.

Draught beer is where the difference becomes visible to travellers, and where American practice is looser than British. There is no federal requirement that a glass sold as a pint contain a pint, and the widely used sixteen-ounce shaker glass holds that volume only when filled to the brim, so a poured beer with a head contains less. Several states have legislated on the point precisely because the imperial system's verified glass has no American equivalent.

Domestic packaging uses it constantly. Milk, cream, ice cream and soft fruit are sold in pint containers, and a donated unit of blood is loosely called a pint although it is about 470 millilitres. Premium ice cream in a pint tub is one of the most recognisable retail formats in the country.

A separate United States dry pint of 551 millilitres exists for produce, and nothing in the word distinguishes it. A pint of cream and a pint of blueberries are therefore different quantities, differing by about sixteen per cent, which is the sort of detail that only becomes visible when someone tries to convert a recipe.

One United States liquid pint equals 473.176 millilitres, 16 US fluid ounces, 0.125 US gallons, or about 0.833 imperial pints.

Information about the Gallon (gal)

The gallon in the imperial system is a unit of volume equal to exactly 4.54609 litres. Its symbol is gal. It contains four quarts, eight pints and 160 imperial fluid ounces, and it is about a fifth larger than the United States gallon, a difference that causes more confusion than any other single discrepancy between the two systems.

Its original definition was elegant. The Weights and Measures Act of 1824 fixed the imperial gallon as the volume occupied by ten pounds of distilled water at sixty-two degrees Fahrenheit under a barometric pressure of thirty inches of mercury. That tied liquid measure directly to the avoirdupois pound, so a gallon of water weighed ten pounds and a pint weighed a pound and a quarter. In 1985 the definition was replaced by the exact metric figure, but the old relationship still holds to within a fraction of a per cent.

Fuel economy keeps it in daily use. British and Irish drivers describe consumption in miles per gallon even though fuel has been sold in litres since the 1990s, and the figure is an imperial one. The same car scores about twenty per cent higher in British miles per gallon than in American, purely because the gallon is larger, which is why cross-Atlantic comparisons of efficiency are meaningless unless the gallon is specified.

Legal use in Britain is now narrow but real. Draught beer and cider must be sold in pints, half pints or third pints, and milk in returnable containers may be sold in pints, which keeps the whole imperial chain alive by keeping its smallest working member alive. Several Caribbean territories still sell motor fuel by the imperial gallon.

The difference from the American gallon is worth stating plainly. One imperial gallon is 4.546 litres and one United States gallon is 3.785, so an imperial gallon is 1.2 United States gallons. Recipes, fuel figures, tank capacities and chemical dilutions all break if the wrong one is assumed, and the error is large enough to matter but small enough to look plausible.

Below it the chain runs quart, pint, gill and fluid ounce, each a quarter or a fifth of the one above rather than a decimal step. That structure was designed for dividing a container by halving it repeatedly, which is a practical merit when pouring and a serious nuisance when calculating.

One imperial gallon equals 4.54609 litres, 8 imperial pints, 160 imperial fluid ounces, or about 1.201 US gallons.