There are 128 fluid ounces in a US gallon. If you are referring to the UK (imperial) gallon, it contains 160 fluid ounces.
Summary:
Measurement System| Ounces per Gallon
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US gallon| 128 fluid ounces
UK (imperial) gallon| 160 fluid ounces
Let me know if you need conversions for dry ounces or other measurements!
