1 Gallon [Fluid, US] = 128 Ounces [Fluid, US]
1 Gallon [Dry, US] = 148.946 Ounces [Fluid, US]
1 Gallon [UK] = 160 Ounces [UK]
There are 128 ounces in a gallon, because from the largest to the smallest units between gallons and ounces, there are 4 quarts in a gallon, 2 pints in a quart, 2 cups in a pint and 8 ounces in a cup, that makes 4 * 2 * 2 * 8 = 128 ounces in a gallon.
For example, to find out how many ounces there are in a gallon and a half, multiply the number of gallons by the conversion factor, that makes 1.5 gal * 128 = 192 ounces in 1.5 gallons.
Gallon and fluid ounce are US customary and imperial (UK) systems volume units. In US customary one gallon equals to 128 fl. ounces and in imperial (UK) system one gallon equals to 160 fl. ounces. The abbreviation for gallon is "gal" and for fluid ounce is "fl oz".
One US fluid gallon is defined as 231 cubic inches and one US dry gallon is 268.8 cubic inches. An imperial gallon is 277.4 cubic inches. One US fluid ounce equals to 29.5 mL and one imperial ounce equals to 28.4 mL.