How many ounces is in a half a gallon?

64 ounces. 1 gallon makes 128 ounces and 1 ounce is 0.0078 gallon in U.S measure. There are 8 ounces in a cup, 2 cups in a pint, 2 pints in a quart and 4 quarts in a gallon. That makes 128 ounces in a gallon US. In UK 20 ounces to the pint, 8 pints to the gallon which makes 160 ounces Imrerial measure.