Convert milliliters to fluid ounces

The formula

fluid ounces = milliliters × 0.033814023

milliliters to fluid ounces — quick reference

The imperial fluid ounce is the smaller one, which is the wrong way round

The US fluid ounce is 29.5735 millilitres and the imperial fluid ounce is 28.4131, so the imperial one is smaller even though the imperial gallon is the larger of the two gallons. The reason is the division rather than the size: an imperial gallon is cut into 160 fluid ounces and a US gallon into 128. The two systems kept the names and disagreed about the fractions.

In practice 500 ml is 16.9 US fluid ounces or 17.6 imperial, and a 750 ml wine bottle is 25.4 US ounces against 26.4 imperial. Neither reading is wrong; the question is who is doing the reading. American labels, recipes and drinks menus mean the US ounce, and it is the one to assume unless the text is British and predates metrication.

Where the four per cent has a consequence: cabin baggage

Airport security caps containers at 100 ml, which American signage renders as 3.4 fluid ounces — a converted number, which is why it carries a decimal at all. A bottle labelled 3 fl oz is 88.7 ml and passes anywhere. A bottle labelled 3.5 fl oz is 103.5 ml and can be taken off you however little is left inside, because the rule counts the container’s stated capacity, not its contents.

Cosmetics run the same trap more gently. A 30 ml serum is sold as 1 fl oz, a rounding of 1.014; a 50 ml jar becomes 1.7 oz, understating by a shade. These are marketing roundings rather than conversions, so comparing price per ounce across brands that round in different directions compares two slightly different quantities.

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