A gallon is a unit of volume in the United States customary system of It is defined as the volume of a liquid that can be held in a container that is one-sixth of a bushel, or about 4.8 liters. There are 16 fluid ounces in a pint, and there are 2 pints in a quart. Therefore, there are 32 fluid ounces in a quart. Since there are 4 quarts in a gallon, the total number of fluid ounces in a gallon is 32 ounces * 4 quarts = 128 fluid ounces.