A gallon contains 128 fluid ounces in the U.S. customary system

. To clarify further:

• 1 U.S. liquid gallon = 128 U.S. fluid ounces
• The Imperial (UK) gallon is larger and equals 160 Imperial fluid ounces

So, if you are referring to the common U.S. measurement, a gallon is 128 ounces.