There are five fifths in a gallon, and there are two and a half fifths in half a gallon. A fifth was a former unit of measurement for alcoholic beverages in the United States.
The fifth unit of measurement is roughly equivalent to 25.6 fluid ounces or 750 millilitres; as of 2014, millilitres are the preferred way for measuring alcohol, including wine in standard-sized bottles. In the United States, wine and spirits were bottled in fifths until 1973, when the metric system began to replace the English system of measurement. Bottles of beer continue to be marked with the total number of ounces, which typically ranges from 12 to 40.