There are 128 fluid ounces in a US gallon. The US gallon is the commonly used measurement in the United States and equates to 128 fluid ounces of volume. It is important to differentiate fluid ounces, which measure volume, from ounces that measure weight. For comparison, the UK (imperial) gallon contains 160 fluid ounces, but in the US system, 1 gallon equals 128 fluid ounces.