There are five fifths in a gallon, and 2.5 fifths in half a gallon. A fifth is a unit of measurement formerly used in the United States for distilled beverages.
The fifth unit of measurement is roughly equal to 25.6 fluid ounces or 750 milliliters; milliliters are the more common modern method of measuring alcohol as of 2014, including wine in standard-size bottles. Wine and distilled liquor were bottled in fifths until 1973, which is the year the metric system began replacing the English measurement system in the United States. Beer continues to be sold in bottles marked with the total number of ounces, generally ranging from 12 to 40.