How this instrument works
This ounce owes its size to English wine barrels. Queen Anne's statute of 1707 fixed that gallon at 231 cubic inches, convenient rounding of a cylinder seven inches across and six deep, whose true capacity works out to 230.907 cubic inches. America kept that vessel after independence; Britain abandoned it in 1824 for its larger imperial gallon of 4.54609 litres. Cut a US gallon into 128 parts and each part is one fluid ounce.
Those cubic inches became metrically exact in 1959, when six standards bodies agreed on an inch of precisely 25.4 mm. Everything else then follows by arithmetic: a cubic inch is 16.387064 cm³, a US gallon 3.785411784 litres, a fluid ounce 29.5735295625 millilitres — every digit final, not one of them measured. Invert that and 1 ml works out to 16000000⁄473176473 of an ounce, or 0.033814022701843… in decimal, endless yet perfectly determined.
Millilitres needed repairs of their own. Before 1964, one litre meant space filled by exactly one kilogram of pure water at peak density, making it 1.000028 cubic decimetres; that year's General Conference on Weights and Measures scrapped such wording, so millilitres and cubic centimetres have been identical ever since. Both units still earn their living: American shelves carry 12 fl oz cans, 1.5 fl oz bar pours and 8 fl oz recipe cups, while millilitres run pharmacy dosing, laboratory glassware, and wine bottled at 750 millilitres since US regulators went metric on wine in 1979.
- Type your volume into Millilitres (ml); 500, one half-litre bottle, is preloaded as your starting point.
- Read your answer on US fluid ounces (floz). It recomputes with every keystroke, no button needed.
- Small doses are welcome: entry steps in thousandths, so 2.5 or 7.5 millilitres go in without fuss.
- Need ounces back to metric? Multiply that ounce figure by 29.5735295625, exact reversal of what this sheet does.
- Round to one decimal for package labels, three for cooking; keep all ten digits only when auditing filling lines.
Worked example — one 500 ml bottle in US ounces
Pick up any half-litre water bottle and read what is printed under its neck: 500 ml, and beside that, 16.9 FL OZ. Enter 500 into Millilitres (ml) and US fluid ounces (floz) returns 16.9070113509 — precisely what that shelf label was rounded from.
Check it by hand: 500 ÷ 29.5735295625 = 16.9070113509. Now run it backwards. Exactly 16.9 fl oz comes to 499.7926 millilitres, about one fifth of a millilitre short of 500, which is why bottling lines target their metric figure and let ounces absorb all rounding.
Questions
Is a US fluid ounce exactly 29.5735295625 ml?
Yes, and no measurement was involved. A US gallon is defined as 231 cubic inches, an inch has been exactly 25.4 mm since 1959, and 128 fluid ounces make that gallon. Multiply those three definitions together and 29.5735295625 millilitres drops out with zero uncertainty. Going from millilitres to ounces reverses it, and reversal is where digits stop terminating — hence 0.0338140227018 shown here, correct to thirteen decimals.
Why does my 500 ml bottle say 16.9 fl oz?
Because 16.9 is 16.9070113509 rounded to one decimal, and US packaging rules ask for both metric and customary net-contents statements on the same panel. Only 0.0070 fl oz separates them, about 0.21 millilitres, well inside tolerances any filler works to. Read it backwards and printed 16.9 fl oz would mean 499.79 ml, which is why that metric number counts as authoritative and its ounce figure as rounded companion.
Is this the same as a UK or imperial fluid ounce?
No, and mixing them is the classic error. An imperial fluid ounce is 28.4130625 millilitres, smaller by 3.92%, since Britain's 1824 gallon of 4.54609 litres divides into 160 ounces rather than 128. Your 500 ml therefore reads 16.907 US fluid ounces but 17.598 imperial ones. Older British, Irish, Canadian and Australian recipes mean imperial; anything American means what this page computes.
Does a fluid ounce weigh an ounce?
Only by coincidence, and not closely enough to trust. An avoirdupois ounce is 28.349523125 g of mass; a US fluid ounce is 29.5735295625 millilitres of volume, which for cool water weighs near 29.6 g — some 4% heavier. Cooking oil fills that same measure at about 27 g, honey at roughly 42 g. Scales and measuring jugs answer different questions, so never swap one figure for another without knowing what substance sits in between.
Why do nutrition labels treat a fluid ounce as 30 ml?
US labelling regulations permit rounded metric equivalents in serving statements, so 30 stands in for 29.5735, and one labelling cup holds 240 millilitres rather than 236.5882365 for customary cups. That is legal convenience for print, roughly 1.4% high, not redefinition. Where accuracy matters — medication, laboratory volumes, standards-of-fill checks — use full factors given above.
How many millilitres sit in common American measures?
One teaspoon is 4.92892159375 millilitres and a tablespoon three times that, 14.78676478125. Cups of 8 fl oz hold 236.5882365, pints 473.176473, quarts 946.352946, all in millilitres. Soda cans of 12 fl oz take 354.88, while 1.5 fl oz spirit pours come to 44.36. Every measure there descends from that same 231-cubic-inch gallon, so each stays exact until you round it for print.