There are 128 fluid ounces in 1 US gallon. This is based on the US customary system where 1 gallon equals 4 quarts, 8 pints, 16 cups, or 128 fluid ounces

. For comparison, the UK (Imperial) gallon contains 160 fluid ounces because the Imperial gallon is larger (about 4.546 liters) than the US gallon (about 3.785 liters)

Summary:

• 1 US gallon = 128 US fluid ounces
• 1 Imperial (UK) gallon = 160 Imperial fluid ounces

This distinction is important depending on which measurement system you are using.