A fifth is a unit of volume

Volume

Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

 formerly used for distilled beverages in the United States, and is equal to one fifth of a gallon

Gallon

The gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use: the imperial gallon which is used in the United Kingdom and semi-officially within Canada, the United States liquid gallon and the lesser used United States dry...

, quart

Quart

The quart is a unit of volume equal to a quarter of a gallon, two pints, or four cups. Since gallons of various sizes have historically been in use, quarts of various sizes have also existed; see gallon for further discussion. Three of these kinds of quarts remain in current use, all approximately...

, or 25 fluid ounce

Fluid ounce

A fluid ounce is a unit of volume equal to about 28.4 mL in the imperial system or about 29.6 mL in the US system. The fluid ounce is distinct from the ounce, which measures mass...

s (757 mL); it has been superseded by the metric fifth of 750 mL, approximately 1% smaller, which is the standard capacity of wine bottle

Wine bottle

A wine bottle is a bottle used for holding wine, generally made of glass. Some wines are fermented in the bottle, others are bottled only after fermentation. They come in a large variety of sizes, several named for Biblical kings and other figures. The standard bottle contains 750 ml,...

s world-wide.

History

In the late nineteenth century, liquor was often sold in bottles which appeared to hold a quart (32 fl.oz.) but in fact contained 2, 3, or 4 fluid ounces less than a quart and were called "fifths", short quarts, or commercial quarts.

A quart or one fifth of a gallon was a common legal threshold for the difference between selling by the drink and selling by the bottle or at wholesale, and thus the difference between a drinking saloon or barroom

Bar (establishment)

A bar is a business establishment that serves alcoholic drinks — beer, wine, liquor, and cocktails — for consumption on the premises.Bars provide stools or chairs that are placed at tables or counters for their patrons. Some bars have entertainment on a stage, such as a live band, comedians, go-go...

 and a dry-goods store.

The fifth was the usual size of bottle for distilled beverages in the United States until 1980. Other authorized units based on the fifth included pint and pint.

During the 1970's, there was a push for metrication

Metrication

Metrication refers to the introduction and use of the SI metric system, the international standard for physical measurements. This has involved a long process of independent and systematic conversions of countries from various local systems of weights and measures. Metrication began in France in...

 of U.S. government standards. In 1975, the Bureau of Alcohol, Tobacco and Firearms, in cooperation with the Distilled Spirits Council of the United States

Distilled Spirits Council of the United States

The Distilled Spirits Council of the United States is a national trade association representing producers and marketers of distilled spirits sold in the United States...

, proposed six metric-standard bottle sizes to take effect in January, 1979, and these standards were incorporated into Title 27 of the Code of Federal Regulations

Code of Federal Regulations

The Code of Federal Regulations is the codification of the general and permanent rules and regulations published in the Federal Register by the executive departments and agencies of the Federal Government of the United States.The CFR is published by the Office of the Federal Register, an agency...

. These sizes are 50, 100, 200, 375 (355 for cans), 500 (until June, 1989), 750, 1000, and 1750 mL.