[laurencerowe]

While ounces are the same, fluid ounces are different!

> An imperial fluid ounce is 1⁄20 of an imperial pint, 1⁄160 of an imperial gallon or exactly 28.4130625 mL. A US customary fluid ounce is 1⁄16 of a US liquid pint and 1⁄128 of a US liquid gallon or exactly 29.5735295625 mL, making it about 4.08% larger than the imperial fluid ounce.

https://en.m.wikipedia.org/wiki/Fluid_ounce

[francisdavey]

Indeed.

Worse, the ounce, as a unit of mass, has two different meanings in English law, one of which is so obscure and differs so very little from the internationally agreed quantity, that it is hard to believe that it could ever matter, but strictly if you are thinking about the law of food labelling - and I do sometimes - then you always have a slight twinge at the thought.

[fanf2]

The US customary fluid ounce is based on the Queen Anne wine gallon of 231 cu in. The Imperial weights and measures reform in the 1820s defined a new gallon equal to the volume of 10 lb avoirdupois of water at s.t.p., which was about the same as the old ale gallon. The Imperial pint was changed to be 20 fl. oz. instead of 16 fl. oz. so that the metric-style correspondence between mass and volume also worked for ounces.