The Pint has been the usual serving quantity for draught beer in the British public house at least since the beginning of the 20th century. In earlier centuries, before the pint became ubiquitous, the “pot,” or quart—equal to 2 pints—was the norm. In Britain today, “going for a pint” has become equivalent to “going for a beer.”
The pint is one eighth of a gallon, and a gallon was originally the volume of 8 lb of wheat. By the 18th century a number of different “gallons” were recognized in Britain, including the “wine gallon,” defined by Parliament in 1707 as equal to 231 in3, and the beer or ale gallon as equal to 282 in3.
The United States adopted the wine gallon of 231 in3 as its standard gallon measure, which made a US pint 28.875 in3 or 473.176 ml. In the United Kingdom the Imperial Weights and Measures Act of 1824 abolished all other gallon measures and brought in the Imperial gallon, equal to the volume of 10 pounds of distilled water at a precise temperature, or 277.419 in3. The Imperial pint, one eighth of this, is thus equal to 34.677 in3 or 568.261 ml.
Both the US and the UK pints are divided into “fluid ounces” each meant to be equal, or approximately equal, to the volume of 1 oz of water at a specific temperature and pressure. The Imperial pint contains 20 British fluid oz equal to 28.413 ml each. The American pint, by contrast, contains 16 US fluid oz equal to 29.574 ml each. This makes the US fluid ounce 4% larger than the Imperial one. The Imperial pint is approximately 20% larger than the US pint.