There are 128 U.S. fluid ounces in a U.S. gallon of water. Furthermore, there are 160 imperial fluid ounces in an imperial gallon. The U.S. ounce and gallon are used in the United States, while the imperial versions of these units of measurements are used in Britain.
The U.S. fluid ounce is sometimes referred to as the customary fluid ounce, to distinguish between it and the U.S. food labelling fluid ounce, which is as per regulation 21 CFR 101.9(b)(5)(viii). This regulation defines the food labelling fluid ounce as exactly equal to 30 milliliters, which is equal to 1.014420681 U.S. customary fluid ounces. The fluid ounce is sometimes just called the ounce, but it is distinct from the ounce that is a unit of measurement for mass.