How many ounces are in a half gallon?
128 ounces in a gallon 128/2= 64 ounces are in a half gallon
77.5151182665 according to http://www.convertunits.com/from/gallon/to/ounces. Are you looking for liquid ounces? It really depends on what you are looking for. 128 fluid ounces in a gallon of water. 64 ounces is half. If you are looking for volume gallon to mass ounces it will vary depending upon the density.