Convert oz to g

An ounce is a sixteenth of a pound. It is not the fluid ounce, which measures volume, and the two are only loosely related — a fluid ounce of water weighs about an ounce, which is where the name came from and where the confusion starts.

Grams are the unit of recipes and postage outside the United States, and the unit nutrition labels use nearly everywhere.

The factor is 28.349523, and almost nobody carries that around. Rounded to 28 it is off by 1.2 % — which stays invisible on small numbers and turns into a whole unit somewhere around 100 oz.

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 ounce has been defined in metric terms since the international yard and pound agreement of 1959, and in this page's units that comes out at 28.3495231 g to one. The definition is exact by stipulation — it was not measured, and it cannot be refined by measuring it better. The decimal above is not the whole of it, though: this page stops at nine significant digits and the ratio carries on past them.

Before that agreement the US and the British versions differed slightly, and old survey figures still carry the older definition. For anything outside land surveying the difference is far below the precision anybody is working to.

American recipes measure most things by volume and a few things by weight, and both use the word ounce. Eight ounces of chocolate is a weight; eight ounces of milk is a fluid ounce figure meaning 236.6 ml. The abbreviation "fl" is dropped so often that its absence proves nothing, and the two are only equal for a substance with the density of water.

The practical rule is to look at what is being measured rather than at the units. Liquids, and anything a cup measure could hold, are volumes. Solids bought by the package — cheese, chocolate, nuts, pasta, meat — are weights, because that is what the package declares. Where a recipe is ambiguous, the cup is the tell: a cup is 8 fl oz, so any quantity that appears elsewhere in cups is a volume everywhere.

A troy ounce is 31.1034768 g against the ordinary ounce's 28.349523125 — 9.71 per cent heavier — and the two share a name and nothing else. It survives because precious metals are priced in it: a quoted gold price is per troy ounce, and a troy pound is twelve troy ounces rather than sixteen, so the pound relationship does not carry over either.

It never appears in a kitchen, and the reason to know it exists is that search results and conversion tables do not always separate the two. A figure of 31.1 g where 28.3 was expected is the signature, and a recipe that produces it has been converted through the wrong table. Nothing sold as food anywhere declares its weight in troy ounces.

It is the packaging, not the cooking. American home cooking measures in cups and spoons, and the ounce enters when an ingredient is bought rather than scooped: a 14 oz can, an 8 oz block, a 12 oz bag. The recipe passes the package size straight through, which is why ounce quantities in American recipes cluster on retail sizes rather than on round numbers.

That is useful when converting, because it tells you the tolerance. A recipe asking for a 14 oz can of tomatoes — 397 g — is asking for "a can", and the nearest metric tin at 400 g is the right answer rather than an approximation. A recipe asking for 3.5 oz of an ingredient measured out on purpose is asking for 99 g and means it.

The two systems chose their pack sizes independently and the results do not line up. A stick of butter is 4 oz, 113 g, against European 250 g blocks. American chocolate comes in 4 and 12 oz bags, 113 g and 340 g, against European 100 g and 200 g bars. A pound of pasta is 454 g against a 500 g packet.

For anything cooked to taste this is noise. For baking it is the difference between a dough that behaves and one that does not, because the ratios matter more than the absolute quantities. The safe approach with a converted recipe is to scale the whole thing to the pack size you actually have — 500 g of flour instead of 454, with every other ingredient multiplied by 1.10 — rather than to use a whole packet of one ingredient and a converted amount of the rest.

Every ounce quantity converts to a gram figure with a decimal in it: 1 oz is 28.3 g, 2 oz is 56.7, 3 oz is 85, 6 oz is 170.1. A domestic scale steps in whole grams, so it can get within half a gram of any of those and no closer, which is an error of under two per cent on a single ounce and under a third of a per cent on six.

That resolution is the real limit on a converted recipe, not the conversion. Where the quantity is small — a third of an ounce of salt, a quarter-ounce packet of yeast — the scale's gram step is a large fraction of the total, and a scale that reads to a tenth of a gram changes the outcome in a way that no amount of decimal places in the conversion can.

Most of a recipe tolerates a few per cent. A handful of ingredients do not: salt in bread sits around two per cent of the flour weight, instant yeast around one, chemical leaveners around one to two. At those proportions the ingredient is measured in single grams, and a rounding that would be invisible on the flour is a tenth of the quantity here.

A quarter-ounce packet of yeast is 7.09 g and is sold in Europe as a 7 g sachet, which is the same thing. A teaspoon of fine salt is about 6 g and of coarse salt about 4, which is why salt is the ingredient most often wrong in a converted recipe — the volume measure the original used depends on the crystal, and the weight does not. Where a recipe gives salt by weight, that figure is worth more than any other line in it.

A recipe converted as you go is a recipe converted at different precisions, and the ratios drift. Doing all the lines first also surfaces the fluid ounces, because they are the ones that will not produce a sensible gram figure for what they are describing — 8 oz of stock coming out as 227 g is the moment to notice it was a volume.

It is also the point at which to decide the batch size. Converted American quantities land on untidy gram figures, and rounding them all in the same direction keeps the recipe intact while rounding them individually to convenient numbers does not. Pick one ingredient to be the round one — usually the flour or the main solid — and scale everything else to it.

Older British cookbooks are full of ounces, and they mean the same 28.35 g unit — but they use it differently. British home cooking weighed things, so an ounce in a British recipe is almost always a weight and the volume measures are pints and spoons rather than cups. Converting one is therefore easier and more reliable than converting an American recipe of the same age.

The trap in those books is elsewhere: the pint. An imperial pint is 568 ml against the US 473, twenty per cent apart, and a British recipe calling for half a pint of milk wants 284 ml where an American reader would supply 237. Where a recipe mixes ounces and pints, the ounces convert with this page and the pints do not convert with anything on it.