How many fl oz in 1 gallon of water?
There three answers - one for UK gallons and fluid ounces, one
for US liquid gallons and US fluid ounces, and a third for US dry
gallons and US fluid ounces. (Of course there are many more,
converting between UK and US units, but this is rare.)
UK: 1 gal. = 160 fl oz
US (liq): 1 gal. = 128 fl oz
US (dry): 1 gal. = 149 fl oz