Many wines and whiskeys use 750 milliliters (ml) as the standard amount of liquid that goes into a bottle. One gallon equals 3,785 ml. 750 ml, the standard bottle, would be about one fifth of that amount. Ergo, a fifth (of a gallon).

A standard bottle of liquor is often called a "fifth" because it originally represented one-fifth of a U.S. liquid gallon. In the late 19th and early 20th centuries, a U.S. gallon was approximately 3.78 liters, so a fifth-gallon bottle contained about 25.6 U.S. fluid ounces (roughly 757 milliliters). This size became the industry standard for distilled spirits in the United States. When the U.S. began transitioning to the metric system for spirits in 1979, the "fifth" was replaced by the 750ml bottle, which is the universal standard used today. Although 750ml is slightly less than a true historical fifth (which was 757ml), the name "fifth" stuck in common parlance as a colloquialism for a standard-sized bottle of whiskey, vodka, or rum. The term serves as a linguistic relic of the old Imperial measurement system, similar to how we still use terms like "pint" or "quart" in non-commercial settings even as the world has moved toward milliliters and liters for official labeling.