There are 128 fluid ounces in 1 US gallon. This measurement applies to the US liquid gallon, which is commonly used in the United States for measuring volume, especially liquids. Note that fluid ounces measure volume, and this differs from ounces which can also measure weight. Additionally, the UK uses a different gallon measurement that equals 160 UK fluid ounces, but the standard US gallon contains 128 fluid ounces.