Gallons to ounces (gal to fl oz)

One US gallon is exactly 128 US fluid ounces. That ratio belongs to the customary system itself and is much older than any link to metric: the 1959 international yard and pound agreement later fixed the yard and the pound against the meter and the kilogram, but it left the ratios between customary units (feet to the yard, ounces to the pound, cups to the gallon) exactly as they were.

Worked through: 1 gal x 128 = 128 fl oz.

Going the other way, one US fluid ounce is 0.0078125 US gallons.

For mental arithmetic: Multiply by 128.

A US gallon is exactly 128 fluid ounces, which is 4 quarts of 32, or 16 cups of 8. The whole US volume system is built in halvings from the gallon down, and every step is a power of two until you reach the tablespoon. That is why the awkward conversions in American cooking are the ones involving thirds, and why none of the ones involving halves and quarters ever need looking up twice. The imperial gallon is 160 imperial fluid ounces, a different number of different ounces.

Precision is worth matching to the input. A measurement given as 0.25 gal carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.