How many ounces in a gallon? How many ounces in a gallon ? If the conversion is from U.S. fluid gallons to ounces then there are 128 ounces in a gallon. The commonly converted numbers and the results can be found in the conversion table at the end of the page but for anything else please use the converter below. 1 Gallon [Fluid, US] = 128 Ounces [Fluid, US] 1 Gallon [UK] = 160 Ounces [UK]