Convert cup to fl oz

A US customary cup is 236.6 ml, but American nutrition labels use a "legal" cup of exactly 240 ml, and a metric cup in Australia is 250. A recipe scaled up tenfold will show which one it assumed.

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.

Going this way is a multiplication, and by a whole number: one US cup is 8 US fluid ounces, exactly, and 8 is the definition rather than a measurement that came close.

That makes it one of the few conversions worth doing in your head, and it makes the answer checkable: divide back and you must land on the number you started with, exactly, with no remainder to explain away.

The US fluid ounce is two different sizes wearing one name. This page uses the US one, which is 0.125 cup; the imperial one still used in the UK and Ireland is 0.12009 cup — 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.

The US customary cup is defined as one sixteenth of a US gallon, and the US fluid ounce as one hundred and twenty-eighth of the same gallon. Eight fluid ounces to the cup therefore falls out of the definitions rather than being measured, which makes this one of the few conversions on this site with no rounding anywhere in it. Two cups is 16 fl oz, three is 24, and there is no decimal to lose.

The gallon those definitions sit on is itself exact: 231 cubic inches, a figure inherited from an English wine gallon and fixed in metric terms once the inch became exactly 25.4 mm. So a cup is exactly 236.5882365 ml, and the exactness runs all the way down. Everything difficult about this conversion happens above the arithmetic, in what the reader is holding.

A measuring jug sold in Australia, New Zealand or much of Europe uses a metric cup of exactly 250 ml. That is 8.45 US fluid ounces, or 5.7 per cent more than the recipe means if the recipe is American. One cup of milk over by 5.7 per cent is nothing; a cake made with four cups of flour measured in metric cups has nearly a quarter cup of extra flour in it, which is the difference between a crumb and a brick.

The tell is on the jug itself. If the cup line sits at 250 ml, it is a metric cup. If it sits at 240 ml, it was made to the US labelling standard. If it sits at 236 or 237 ml, it is the customary cup the conversion above assumes. Where the jug carries no millilitre scale at all, filling it with water and weighing the result settles the question in one step, since a millilitre of water weighs a gram.

US food labelling defines a cup as exactly 240 ml for the purpose of stating serving sizes. That is 8.12 US fluid ounces — 1.4 per cent above the customary cup — and the reason it exists is arithmetic convenience: 240 divides neatly and 236.588 does not. A label that says a serving is one cup means this one.

The practical risk is not the 1.4 per cent, which nothing in a kitchen can detect. It is that three cups are now in circulation with the same name, and a recipe writer who took a figure off a label, a jug and a cookbook has silently mixed all three. When precision matters — baking, brining, anything scaled up tenfold — the fix is to leave cups behind entirely and work in grams or millilitres.

Both are 8 fluid ounces; the difference is entirely in how they are read. A liquid measuring jug puts its top graduation below the rim so that a full cup can be carried without slopping, and it is read at eye level against the bottom of the meniscus. A dry measuring cup has no headroom at all: it is filled past the rim and levelled with the back of a knife, and there is no line to read.

Using the wrong one costs more than it looks. Levelling flour in a liquid jug is impossible because the graduation is below the rim, so the cook fills to a line by eye and typically overshoots. Pouring liquid into a dry cup to the brim and then moving it loses some of it. Neither error is in the conversion, and both look exactly like the conversion being wrong.

The 8 in this conversion is 8 fluid ounces, a volume. A cup of all-purpose flour weighs a little over 4 ounces, a cup of granulated sugar around 7, a cup of honey close to 12. Only water and things near its density make the volume figure and the weight figure agree, which is the historical accident the shared word records.

This is the point at which US and European recipe writing part company. A European recipe gives flour in grams because the same volume of flour can vary by a fifth depending on whether it was scooped or spooned into the cup. An American recipe gives cups and accepts the variation. Converting cups to fluid ounces preserves the volume exactly and preserves none of that uncertainty about what is in the cup.

A jug read from above shows more liquid than it holds, because the eye follows the near rim down to the far side of the surface. On a two-cup jug graduated in 2 fl oz steps, that parallax is comfortably worth half an ounce, and it is consistent — every pour is wrong in the same direction, so repeated measures accumulate rather than cancel.

Small measures are worse than large ones for the same reason a small ruler is worse: the fixed error is a bigger share of the total. Measuring 4 cups as four separate cupfuls carries four readings of error, where a four-cup jug read once carries one. Where a recipe needs a quart, measure the quart.

The cup as a recipe unit is largely American, with Australia, New Zealand and Canada using a metric version of it. British and most European recipe writing gives dry ingredients by weight and liquid in millilitres, which is why a British cookbook rarely needs converting at all and an Australian one always does — the word is the same and the vessel is 250 ml rather than 236.6.

Japan runs a third convention worth knowing about if a recipe arrives from there. The general-purpose Japanese cup is 200 ml, and the cup supplied with a rice cooker is smaller again at 180 ml — one gō, an old unit of rice that survived into modern appliances. A rice recipe measured in cooker cups and converted as though they were US cups is out by nearly a quarter.

The cup sits one rung above the fluid ounce and two below the quart, and the jug in most kitchens is calibrated to show it. A two-cup jug is a pint, a four-cup jug is a quart, and the 16 and 32 fluid ounce marks on their sides are the same lines under a different name. Recognising that saves converting at all for the common cases.

Above a quart the cup stops being a sensible unit. Sixteen cups make a gallon, and measuring a gallon in cupfuls is sixteen chances to misread a line. Once a quantity passes about a quart, the useful move is to step up the ladder — count quarts, or weigh — and reserve the cup for the amounts a hand can lift in one go.