Full answer
The international avoirdupois ounce — the standard used in US cooking and everyday weight measurements — equals exactly 28.3495 grams. For kitchen purposes, rounding to 28.35g or simply 28g is accurate enough for any recipe.
When scaling recipes between US and metric measurements, a few anchor points are worth memorizing: 1 ounce is approximately 28g, 4 ounces (a quarter pound) is 113g, 8 ounces is 227g, and a full pound (16 oz) is 454g. These round numbers cover the vast majority of ingredient quantities you'll encounter.
Note that the fluid ounce, used for liquids, measures volume rather than weight. One US fluid ounce is 29.57 mL — close to 28.35g only if the liquid is water (density ≈ 1 g/mL). For heavier liquids like honey or oils, the weight per fluid ounce differs significantly.
There is also the troy ounce (31.1 grams) used exclusively for precious metals like gold and silver. If you see an ounce price quoted for gold, it is always the troy ounce — never the cooking ounce. Context almost always makes it clear which system applies.