There are 128 fluid ounces in one gallon of water in the system of measurement used by the United States. By definition, a fluid ounce is 1/16 of a pint, and there are eight pints in a gallon.
The term "ounce" has several different meanings. Fluid ounces measure volume, while the term "ounce" sometimes indicates weight. A gallon of water at 70 degrees Fahrenheit weighs 8.33 pounds or 133.28 ounces. However, the density of water changes due to temperature. Water heated to the boiling point weighs exactly 8 pounds or 128 ounces. Customarily, fluid ounces apply to liquids, while weights in ounces apply to dry materials.