Ounces to grams
One ounce is exactly 28.349523125 grams. Exactly, because the pound is defined as exactly 0.45359237 kilograms and an ounce is a sixteenth of it, so the figure is a definition rather than a measurement. For cooking, 28.35 is more precision than any kitchen scale offers, and US food labels are allowed to round it to 28.
How to convert ounces to grams
Three different things are called an ounce, and picking the wrong one is expensive in two of the three cases.
The avoirdupois ounce is the everyday one, and the one converted here. It is a sixteenth of a pound, or 437.5 grains, or 28.349523125 grams, and it covers flour, parcels, boxing gloves, breakfast cereal and everything else sold by weight in the US.
The troy ounce is 480 grains, 31.1034768 grams, about 9.7 per cent heavier, and it exists only for precious metals. A gold or silver price quoted per ounce is per troy ounce without saying so. Converting a bullion weight with the everyday factor understates it by nearly a tenth, and at any realistic gold price that is a large number. The troy system also has its own pound, of twelve ounces rather than sixteen, so a troy pound is lighter than an ordinary one even though its ounces are heavier.
The fluid ounce is a volume and not a weight at all, so converting it to grams needs the density of whatever is in the container. Water is close to 29.6 grams per US fluid ounce, honey nearer 42, oil around 27. Worse, the fluid ounce splits by country in the opposite direction to what people expect: the imperial fluid ounce is 28.41 ml and the US one is 29.57 ml, so the imperial fluid ounce is the smaller of the two even though the imperial pint and gallon are both larger than their US namesakes. A recipe that says "8 fl oz of milk" means 237 ml in an American book and 227 ml in a British one.
One small note on precision. The panel shows four decimal places, which is far past what any domestic scale can resolve. A kitchen scale reads to a gram or occasionally to a tenth, a jeweller's scale to a hundredth, so the digits beyond that are arithmetic rather than information.
What people use it for
- Weighing an American recipe's ingredients on a metric scale
- Checking a metal price quoted per troy ounce
- Converting the net weight printed on a US label
- Working out a coffee dose given in ounces
- Reading a postage weight bracket in grams
- Comparing two products where one is priced per ounce and the other per 100 g
Questions
Exactly 28.349523125, which follows from the pound being exactly 0.45359237 kg. Use 28.35 for cooking.
Because the 1959 international agreement defined the pound as a fixed number of kilograms. Everything below it, including the ounce, is arithmetic from that definition.
Yes. 31.1034768 grams, about 9.7 per cent heavier than the everyday ounce, and used only for precious metals.
The troy ounce, always, and usually without saying so. Using 28.35 instead of 31.1 undervalues the metal by nearly a tenth.
No, lighter. It holds twelve troy ounces rather than sixteen avoirdupois ones, so it comes to about 373 g against 454 g.
Not without knowing the liquid. A fluid ounce is a volume. Water is about 29.6 g, honey about 42, oil about 27.
No, and it is the smaller of the two: 28.41 ml against 29.57 ml. The imperial pint and gallon are larger than the US ones, so this catches people out.
Sixteen, which is 453.59237 grams. In the troy system it is twelve, which is a different pound.
About 3.527. It is the conversion to keep in your head for supermarket comparisons.
Because half a gram of flour changes nothing, and US nutrition labelling is written that way: the regulation says one ounce in weight means 28 grams for labelling purposes.
28 is fine for anything you would measure by eye. Use the full figure for bread, patisserie and coffee, where a two per cent error compounds across the batch.
The unit both ounce systems are built from, 64.79891 mg exactly. An avoirdupois ounce is 437.5 grains and a troy ounce is 480.
4 ounces, which is 113.4 grams before cooking. The weight on the packaging is nearly always the raw weight.
Weight per unit area for fabric, usually ounces per square yard, and total weight for boxing gloves. Neither converts to grams without knowing the area or the pair.
Yes. The direction is kept in this browser, so the page opens the way you last left it.
No. It is one multiplication and it happens as you type.