How to convert ounces in a gallon?

How to convert ounces in a gallon?

Go with the basic fluid-ounce measure in the USA; 8 fl. oz. == 1 cup. Then 4 cups == 1 quart, and 4 quarts == 1 gallon. So, start with the gallon and keep multiplying by 1, with the units attached. I’ve given three such factors:
1 == (4 qt.)/(1 gal.)
1 == (4 cups)/(1 qt.)
1 == (8 fl. oz.)/(1 cup)
Multiply them all together, cancelling units that are repeated both above and below the fraction line, and you get
1 == (128 fl. oz.)/(1 gal.)
That gives you your answer pretty directly.