Fluid ounce

The fluid ounce is a volume unit, distinct from the weight ounce that’s 28.35 grams. Two definitions in active use:

• US fluid ounce: 29.5735 ml (1/128 of a US gallon)
• UK imperial fluid ounce: 28.4131 ml (1/160 of an imperial gallon)

The 4% gap matters when converting British recipes to US units or vice versa. A “pint” is 16 US fl oz (473 ml) or 20 UK fl oz (568 ml) — same word, 20% different volume.

Confusingly, both gallons descend from the medieval English Queen Anne wine gallon (US) and the imperial gallon (UK, defined in 1824) — siblings that diverged in different colonial paths. The US kept the older value; the UK adopted the new metric-aligned imperial gallon.

Convert via our volume converters; the fluid-ounce option defaults to US.