In US customary units, a fluid ounce of water does not weigh an ounce avoirdupois, because US volume measures are not Imperial units: they are based on the pre-Imperial Queen Anne wine gallon.
The British fluid ounce was defined to be the volume of an ounce of water, and the Imperial gallon was defined to be the volume of 10lb of water. This vague nod towards decimalization happened to match the pre-Imperial beer gallon fairly well, and it’s also why the Imperial pint has 20 fl oz instead of 16.
The British fluid ounce was defined to be the volume of an ounce of water, and the Imperial gallon was defined to be the volume of 10lb of water. This vague nod towards decimalization happened to match the pre-Imperial beer gallon fairly well, and it’s also why the Imperial pint has 20 fl oz instead of 16.