A fluid ounce (as opposed to a weight, the avoirdupois ounce) is a unit of volume in the English system of measurement. An ounce is 1/8 of a cup, or 1/16 of a pint, or 1/32 of a quart. It is only tangentially related to the weight ounce, and the two are not directly convertible.
The "imperial ounce" (UK) differs slightly from the US ounce, because they are based on two different reference volumes. The imperial ounce is about 28 mL (28.41 mL) while the US ounce is about 30 mL (29.57 mL).
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