The butt was a measure of liquid volume equalling two hogsheads. This equated to 108 imperial gallons (490 l) for ale or 126 imperial gallons (570 l) for wine (also known as a pipe).

conversion table

A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl., old forms ℥, fl ℥, f℥, ƒ ℥) is a unit of volume (also called capacity) typically used for measuring liquids. Various definitions have been used throughout history, but only two are still in common use: the British Imperial and the United States customary fluid ounce. An imperial fluid ounce is  ¹⁄₂₀ of an imperial pint,  ¹⁄₁₆₀ of an imperial gallon or approximately 28.4 ml. A US fluid ounce is  ¹⁄₁₆ of a US fluid pint and  ¹⁄₁₂₈ of a US liquid gallon or approximately 29.57 ml, making it about 4% larger than the imperial fluid ounce.

US food labelling fluid ounce

US regulation 21 CFR 101.9(b)(5)(viii) also defines a fluid ounce as exactly 30 millilitres, but this is for use in nutrition labelling only.

conversion table