A fifth of liquor is the colloquial term for a 750-milliliter bottle. Converting 750 milliliters to ounces yields 25.4 ounces, just under one-fifth of a 128 ounce gallon, hence the colloquialism. Other traditional bottle sizes, the pint and half-gallon, have also changed with the introduction of metric measures.
Pint bottles are now 375-milliliter bottles, almost 4 ounces less than a true pint. A half-gallon bottle, also known as a handle, is 1.75 liters, also 4 ounces shy of its original measure. Other common measures of liquor are a 1.5-ounce shot, a 50-milliliter mini-bottle, and a 33.8-ounce, or 1-liter, quart bottle.