There are 128 US fluid ounces in one US liquid gallon. This is the standard conversion used in the United States customary system

. To summarize:

• 1 US gallon = 128 US fluid ounces
• 1/2 gallon = 64 fluid ounces
• 1/4 gallon = 32 fluid ounces

Note that the UK (Imperial) gallon differs and contains 160 Imperial fluid ounces, but for the US gallon, the conversion is consistently 128 fluid ounces