How To Convert Ounces To Grams

An ounce of mass is exactly 28.349523125 grams, and that figure is exact rather than rounded. The chain is short: the international avoirdupois pound is defined as 0.45359237 kilograms, and there are sixteen ounces in a pound, so one ounce is 0.45359237 divided by 16, which is 0.028349523125 kilograms, or 28.349523125 grams. Nothing in that reasoning is approximate, which is why the factor carries so many digits. Eight ounces therefore comes to 8 x 28.349523125 = 226.796185 grams, and sixteen ounces comes to 453.59237 grams, which is one pound exactly. Memorise the number in full, because truncating it is the only way to introduce error into an otherwise exact conversion.

Because the factor is exact and there is no offset, the conversion is a single multiplication. That separates it from temperature, where you both scale and shift, and it is worth stating plainly because the two are often mixed up. Ounces and grams share a zero: zero ounces is zero grams, and no mass is left over on either side. The relationship is therefore proportional, so the whole task can be read as a proportion with a fixed ratio. That is also why a round trip closes exactly rather than merely coming close, which is what makes the check at the end of this page reliable.

The word ounce covers three quite different quantities, and this is the single greatest source of confusion. The avoirdupois ounce is a unit of mass, equal to 28.349523125 grams, and it is the one used for body weight, food and parcels. The fluid ounce is a unit of volume, equal to 29.5735295625 millilitres in the United States and 28.4130625 millilitres in the United Kingdom. The troy ounce is a unit of mass again, but a heavier one at 31.1034768 grams, and it is reserved for precious metals. Before converting anything, decide which ounce you actually have, because two of the three are not even the same kind of quantity.

The avoirdupois system divides a pound into sixteen ounces and a pound into seven thousand grains, so one ounce is 437.5 grains. A single grain is 64.79891 milligrams, and multiplying 437.5 by 64.79891 milligrams returns 28.349523125 grams, confirming the factor from the other direction. This is the ounce of bathroom scales, recipe books and postal rates, and it is the only ounce this page converts. If a label says simply ounces and refers to a solid or to a body weight, it means the avoirdupois ounce. When in doubt, ask whether the quantity could sensibly be weighed, because that test separates mass from volume.

A fluid ounce measures volume rather than mass, so no single gram figure can replace the factor above. In the United States a fluid ounce is 29.5735295625 millilitres, so eight fluid ounces is 236.5882365 millilitres. In the United Kingdom the imperial fluid ounce is smaller, at 28.4130625 millilitres, and eight of them make 227.3045 millilitres. Converting a fluid ounce to grams requires the density of the liquid, not a fixed constant: water is close to one gram per millilitre, so eight US fluid ounces of water weighs about 236.6 grams, but the same volume of honey weighs considerably more. That is why a recipe written in fluid ounces cannot be converted to grams by multiplying by 28.349523125.

The troy ounce is the unit used to price gold, silver and other precious metals, and it is heavier than the avoirdupois ounce. It is defined as 31.1034768 grams exactly, or 480 grains, against the 437.5 grains of the avoirdupois ounce. The ratio between them is therefore 31.1034768 divided by 28.349523125, which is about 1.097142857, so a troy ounce is roughly ten per cent heavier. A one hundred gram gold bar is 100 divided by 31.1034768 = 3.2150746 troy ounces. Quoting a precious metal price per ordinary ounce instead of per troy ounce understates the mass by nearly a tenth, which is a costly mistake at these values.

Going the other way means dividing by 28.349523125, or equivalently multiplying by 0.03527396194958041. A two hundred gram portion is therefore 200 x 0.03527396194958041 = 7.054792389916082 ounces. The reverse direction is where careless errors cluster, because multiplying by 28.349523125 when you meant to divide makes the number grow when it must shrink. The giveaway is the magnitude: grams are always the larger number, so if your ounce figure comes out bigger than the gram figure, the factor went the wrong way. A round trip settles it in one line, since dividing the grams by 28.349523125 returns the ounces you began with.

On the metric side the work is decimal, which is the practical reason for converting at all. One ounce is 0.028349523125 kilograms, so eight ounces is 0.226796185 kilograms, and sixteen ounces is 0.45359237 kilograms. Moving between grams and kilograms only shifts the decimal point three places, so 226.796185 grams is 0.226796185 kilograms with no rounding involved. A kilogram is 1000 grams by definition, and the kilogram is the base unit of mass in the International System of Units. Convert once into grams, then shift the decimal point for whatever metric unit you need, rather than converting separately for each one.

Finally, let the precision of the answer follow the precision of the input. A quantity written as 8 ounces is a round number, and although 8 x 28.349523125 = 226.796185 grams, reporting all six decimal places implies an accuracy the measurement never had. Carry the full factor through every intermediate step so that rounding never accumulates, then round once at the end to something the input can support. In a kitchen the honest answer is about 227 grams, while in a laboratory weighing to the milligram the exact figure is meaningful. Round-tripping the result remains the cheapest check: converting 226.796185 grams back gives 8 ounces, so the factor was applied once and in the right direction.