The exact figure
The avoirdupois ounce is one sixteenth of the international pound, so its exact gram value falls straight out of the 1959 pound definition: 0.45359237 divided by 16. It is the ounce meant in cooking, postage, and any ordinary retail context.
A pound is 453.59237 g, which is why 454 g appears constantly on packaging as a rounded metric equivalent of one pound, and why a 500 g pack is slightly more than a pound rather than slightly less.
The troy ounce is a different unit
Gold, silver, platinum and palladium are traded by troy ounce, which is 31.1034768 g — about 9.7% heavier than the ordinary ounce. This is not a rounding variation. It is a separate system with its own pound of twelve troy ounces rather than sixteen, meaning a troy pound is actually lighter than an ordinary pound despite each ounce being heavier.
The financial consequence is direct. A gold price quoted per ounce means per troy ounce, so converting it with the avoirdupois factor overstates the quantity you get by nearly ten percent. Anyone valuing scrap gold from a weight taken on a kitchen scale needs to divide grams by 31.103, not 28.350. The converter above deliberately uses the avoirdupois figure, because that is the correct one for the food and postage cases most people arrive with.
Fluid ounces are volume, not weight
A separate and very common confusion: fluid ounces measure volume and ounces measure mass, and they are not interchangeable. Eight fluid ounces of water happens to weigh about 8.3 ounces, which is close enough to have created the false impression that the units are the same thing.
For anything other than water the relationship breaks down entirely, since it depends on density. Eight fluid ounces of honey weighs about twelve ounces; eight fluid ounces of flour weighs about four and a half. Recipes that mix the two units without saying which they mean are a recurring source of failed baking. The ml to fluid ounces converter handles the volume side.
Why baking by weight wins
Professional and European recipes specify grams because mass is unambiguous while volume is not. A cup of flour can vary by 20% or more depending on whether it was scooped or spooned and how firmly it settled, which is enough to change a cake's texture noticeably.
Grams also scale cleanly: halving 375 g is arithmetic, while halving three-quarters of a cup is a small puzzle. If you are converting an American recipe to weights, the cups to grams converter handles the ingredient-specific densities that make that conversion possible at all.
Frequently asked questions
How many grams in an ounce?
Exactly 28.349523125 g for the ordinary avoirdupois ounce used in food and postage. The troy ounce used for precious metals is 31.1034768 g, about 9.7% heavier.
Is a troy ounce the same as a regular ounce?
No. A troy ounce is 31.1034768 g against the ordinary 28.35 g. Gold and silver prices are quoted per troy ounce, so using the ordinary factor overstates the quantity by nearly ten percent.
Are fluid ounces and ounces the same?
No — fluid ounces measure volume and ounces measure mass. They coincide roughly for water only. Eight fluid ounces of honey weighs about twelve ounces, and of flour about four and a half.