A fifth is a unit of volume formerly used for distilled beverages in the United States, and is equal to one fifth of a gallon, ⁴⁄₅ quart, or 25³⁄₅ fluid ounces (757 mL); it has been superseded by the metric fifth of 750 mL, approximately 1% smaller, which is the standard capacity of wine bottles world-wide.

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 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 ⁴⁄₅ pint and ¹⁄₁₀ pint.

During the 1970's, there was a push for metrication 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, 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. These sizes are 50, 100, 200, 375 (355 for cans), 500 (until June, 1989), 750, 1000, and 1750 mL.

Fifth is the ordinal form of the number five.

Fifth may refer to:

Unit may refer to: