There are 128 ounces in a gallon. A gallon is a unit of measurement for volume, and it is equal to 128 fluid ounces. This means that there are 128 ounces of liquid in a gallon-sized container. One gallon is also equivalent to 16 cups or 4 quarts, so you can use these conversions to determine the number of ounces in a given amount of liquid. For example, if you have 2 quarts of liquid, you can multiply 2 by the conversion factor of 4 to determine that you have 8 cups, or 64 ounces, of liquid.