Convert ml to fl oz

A millilitre is exactly one cubic centimetre, which is why medicine and laboratory work quote both and mean the same thing.

A US fluid ounce is 29.57 ml; a UK one is 28.41 ml. They are different units with the same name, and recipes rarely say which they mean.

The factor is 0.033814, and almost nobody carries that around. Rounded to 0.034 it is off by 0.55 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 ml.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

The US fluid ounce is two different sizes wearing one name. This page uses the US one, which is 29.57353 ml; the imperial one still used in the UK and Ireland is 28.41306 ml — smaller by 3.9 %.

That gap is the awkward size: far too big to ignore in a recipe, and far too small to spot from the answer alone. If your figure came from a British source, take the second number, and note that the fluid ounce is the one unit here where the imperial measure is the smaller of the two.

Airport liquid limits are the commonest reason a millilitre figure has to become fluid ounces, and the two numbers on the sign are the same number written twice. 100 ml is 3.3814 US fluid ounces, and the American rule rounds up to 3.4 oz. A bottle labelled 3.4 fl oz holds about 100.5 ml, fractionally over the metric limit, and no security lane measures it — the printed size is what is read.

That rounding is why a European 100 ml travel bottle and an American 3.4 oz one sit in the same bag without argument. The disagreements start above that size. A 200 ml bottle is 6.76 fl oz and gets labelled 6.8; an American 8 fl oz bottle is 237 ml and is over the limit under either system. Convert first and compare the printed sizes second, because the labels round in different directions.

This page converts the US fluid ounce, 29.5735295625 ml, because that is what an American form, listing or shelf label means. The imperial fluid ounce still used in the UK and Ireland is 28.4130625 ml — smaller by 3.9 per cent. That is the opposite direction to every other unit in the imperial set: the British pint, quart and gallon are all about a fifth larger than the American ones, and only the ounce runs the other way.

The reason is arithmetic rather than history repeating itself. An imperial pint is divided into twenty fluid ounces and a US pint into sixteen, so the larger pint produces the smaller ounce. Anyone applying the "British measures are bigger" rule to an ounce figure gets a number that is wrong by four per cent in the confident direction.

The imperial fluid ounce was constructed to be the volume of one avoirdupois ounce of water. The imperial gallon was defined as ten pounds of water and divided into 160 fluid ounces, so one imperial fluid ounce of water weighs one ounce by design. That is a tidy relationship, and it belongs to the system this page is not using.

The US fluid ounce descends from a different ancestor — a gallon of 231 cubic inches, taken from an English wine measure — and kept the name without the property. A US fluid ounce of water weighs about 1.04 ounces. One word covering both a weight and a volume is where this whole family of mistakes begins, and the two are related only by an accident that stopped being true.

The sizes a metric market sells in do not land on round American numbers, which is why converted labels look awkward. 330 ml is 11.16 fl oz, 500 ml is 16.91, 750 ml is 25.36, and a litre is 33.81. None of those is a size an American producer would have chosen, and a package carrying one of them was designed to a metric number first and translated afterwards.

It works the same way in reverse. A US 12 fl oz can is 355 ml, a 16 fl oz can is 473 ml, and a 20 fl oz bottle is 591 ml — all untidy in metric and all obvious in ounces. Wine is the exception on both sides: 750 ml is the standard bottle in the United States as well, a metric size that customary labelling absorbed rather than replaced.

Four decimal places are more than a label needs and more than any jug can show, so nearly every published ounce figure is rounded to one. 100 ml becomes 3.4, 250 ml becomes 8.5, 500 ml becomes 16.9. The error is under half a per cent, which is smaller than the tolerance a bottling line works to and invisible in anything poured.

It stops being invisible once the figure is multiplied. 250 ml is 8.4535 fl oz, so a recipe scaled twelvefold from a rounded 8.5 is over by more than half an ounce — most of a tablespoon of whatever it was. Where a number will be multiplied, convert at full precision and round at the end, which is the reverse of what is convenient.

An American package often carries an ounce figure this conversion does not apply to. "16 fl oz" is a volume and becomes 473 ml; "Net Wt 16 oz" is a mass and becomes 454 g. The abbreviations differ by two letters, sit in the same corner of the packaging, and are set in the same size of type.

Cosmetics show it most often, because a jar of cream is sold by weight and a bottle of toner by volume, and one brand prints both. Converting a net-weight ounce as though it were fluid produces an answer wrong by whatever the density is — and for anything close to water it lands near enough to look right, which is what keeps the mistake alive.

American marketplaces, shipping forms and product feeds ask for fluid ounces and will not accept millilitres, and a field expecting a number rarely states which ounce it means. It means the US one. Entering an imperial figure understates the volume by about four per cent — small enough to pass validation, large enough to be a misdescription of what is in the box.

Two habits make this survivable. Give the metric figure alongside wherever the form allows free text, because that is the number moulded into the container and the one a customer can check. And keep the decimal: 16.9 fl oz is a true statement about a 500 ml bottle, while 17 fl oz is out by 3 ml and reads as an American size that the container is not.

Consumer packaging sold in the United States is required to declare the net quantity in both customary and metric units, which is why an American bottle of shampoo says 16 fl oz and 473 ml on the same line. The customary figure is the one the shelf and the shopper work in, and the metric one is there because the rule says so.

That is worth knowing before doing the conversion at all: for a great many products the answer is already printed, in smaller type, next to the number that caught the eye. Where both appear and disagree in the last digit, the disagreement is rounding on the manufacturer’s side, and either figure is a fair description of what is in the container.