How many ounces in a gallon? There is a total of 128 ounces in a gallon of water. A gallon of water means drinking 16 eight-ounce glasses. To know more visit the below Links-: How Many Ounces In a Gallon how many oz in a gallon how many ounces in a half gallon how many ounces in 1 4 cup ounces in a gallon oz in a gallon oz to cups