This conversion is a definition, not a measurement
Most conversions between measuring systems are approximations that got better as instruments improved. This one is not. In 1959 the national standards bodies of the English-speaking countries agreed to define the pound as exactly 0.45359237 kilograms, and an ounce is one sixteenth of a pound.
So an ounce is 28.349523125 grams, precisely, forever. There is no measurement that could refine it, because it is not the result of one. The number is a decision.
The formula
grams = ounces × 28.349523125
You will often see 28.35 instead, and for anything you weigh in a kitchen that is fine: the rounding is about two parts in a hundred thousand, which no domestic scale can detect. This calculator uses the full figure anyway, because there is no reason not to.
Worked example
A 500 gram bag of flour.
500 ÷ 28.349523125 = 17.637 ounces
Which is easier to hold in your head as 1 pound 1.64 ounces, since 453.59237 grams make a pound and the remainder is 46.4 grams, or 1.64 ounces.
Going the other way, a 16 ounce pound of butter is 16 × 28.349523125 = 453.59 grams, which is where that awkward-looking 453.59237 comes from: it is the pound itself.
The troy ounce, and why gold is not measured in ordinary ounces
There is a second ounce, and it is worth knowing about before you value anything precious.
A troy ounce is 31.1034768 grams, about 9.7 percent heavier than the everyday avoirdupois ounce this page converts. It is a survivor of the medieval troy weight system, and it is still the unit that gold, silver, platinum and palladium are priced in.
So when a headline says gold is trading at a price per ounce, it means per troy ounce. Convert a gold bar using the ordinary factor and you will understate what you have by nearly a tenth, which on any meaningful quantity is a great deal of money:
| 100 grams of | Ordinary ounces | Troy ounces |
|---|---|---|
| Flour, or anything else | 3.527 | |
| Gold | 3.215 |
The same 100 grams, and two answers nearly a third of an ounce apart. This calculator flags the troy figure alongside the ordinary one whenever you enter an amount, so the choice is visible rather than assumed. If you are actually valuing metal, the gold calculator does the pricing.
Reading a recipe across the Atlantic
American recipes lean on volume: cups, spoons, and ounces that might be either weight or fluid. British and most professional recipes lean on weight, in grams. Neither is wrong, but they fail differently.
Weight is the more reliable of the two, and not by a small margin. A cup of flour can vary by around a quarter depending on whether you scooped it or spooned it, and no amount of care with the cup removes that. A gram is a gram however you got it into the bowl.
If you are converting a recipe rather than a single number, the cups to grams converter carries the per-ingredient densities that make cups and weights line up, and the kg to lbs converter handles the larger end of the same scale.