The U.S. fluid ounce is (1/128) of a U.S. fluid gallon, about 1.805 cubic inches or exactly 29.573 529 562 5 ml. This volume of water weighs about 1.04 avoirdupois ounces.
U.S. law 21CFR101.9(b)(5)(viii) also defines a fluid ounce of exactly 30 ml, for use in nutrition labeling.