US vs UK fluid ounces — the recipe conversion trap
US fluid ounce is 4% larger than UK's. But a US pint is 16 fl oz; a UK pint is 20 fl oz. So US pint (473 mL) is 20% smaller than UK pint (568 mL).
The single most confusing everyday unit family in English measures. US and UK fluid ounces are close in size, but the numbers of ounces in higher units differ, producing a 20% divergence in pints, quarts and gallons.
The exact factors
1 US fl oz = 29.5735 mL > 1 UK (Imperial) fl oz = 28.4131 mL > 1 US pint = 16 US fl oz = 473.176 mL > 1 UK pint = 20 UK fl oz = 568.261 mL > 1 US gal = 3.785 L; 1 UK gal = 4.546 L (20% larger)
Where the error appears
Cocktail recipes (US convention). Beer serving sizes (UK pint on tap in UK, US pint elsewhere). Auto fluid capacities in older UK spec sheets. Baking recipes crossing the Atlantic.
Worked example — baking scale-up
US recipe: 2 cups milk = 16 fl oz = 473 mL. UK cook reads '16 fl oz' and pours 16 Imperial fl oz = 455 mL. Off by 4% — enough to shift batter viscosity on a delicate recipe. Doubled to 1 gallon, the error is 761 mL — the difference between filling a jug and overflowing.
Recap
US fl oz ≈ UK fl oz. But everything above (pint, quart, gallon) diverges 20% because the unit ratios differ.
Try the converters mentioned in this article
FAQ
Which system does a 'cup' use?+
US 'cup' = 8 US fl oz = 237 mL. Metric 'cup' = 250 mL. UK 'cup' historically 10 UK fl oz = 284 mL (rarely used in modern UK recipes).
Are UK litres and US litres the same?+
Yes — the litre is universal (1 L = 1000 mL exactly).
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.