To figure out how many ounces are in a half gallon, first you need to know that 2 quarts are a half gallon. There are 4 cups per quart, and 8 ounces per cup. So, take 8 ounces and multiply by 4, which gives you 32 ounces per quart. Now multiply 32 by 2 which gives you 64. So, there is 64 ounces in a a half gallon.