Sixty-four fluid ounces is equivalent to half a gallon. One fluid ounce is equivalent to 1/128 of a gallon, or .0078125 gallons; therefore, 128 ounces equals exactly one gallon.
For centuries, the fluid ounce did not have a consistent measurement and varied depending on the type of fluid being measured. For example, an ounce of wine was different in volume than an ounce of water. In the early 19th century, the British Parliament defined the imperial fluid ounce as 10 avoirdupois pounds of water, a French unit of liquid measurement.
The U.S. adopted the liquid volume that measures the wine gallon from the French as the standard U.S. liquid gallon. The U.S. liquid gallon is legally defined as 231 cubic inches, weighing about 8.34 pounds, and is exactly fluid 128 ounces.