How many grams are in one ounce?

One ounce, in the context of mass or weight, is precisely 28.349523125 grams. This exact conversion factor is defined by the international avoirdupois ounce, which is the standard unit used in the United States customary system and, for many practical purposes, in countries officially using the metric system when referencing non-scientific measurements like food or postal items. It is critical to distinguish this from the troy ounce, used predominantly for precious metals, which is equivalent to 31.1034768 grams. The avoirdupois ounce is the default meaning in most general and commercial contexts outside of gemology and bullion markets, making the 28.35-gram figure (when rounded) the essential conversion for cooking, shipping, and everyday use.

The establishment of this specific gram equivalence is not arbitrary but stems from the formal redefinition of the international yard and pound agreement of 1959. Prior to this, slight variations existed between the British and American measurement systems. The agreement standardized the international avoirdupois pound as exactly 0.45359237 kilograms. Since one pound is defined as 16 ounces, the gram weight of an ounce is derived by dividing the kilogram equivalent of a pound by 16 and then multiplying by 1000: (0.45359237 kg / 16) * 1000 = 28.349523125 grams. This precise, if cumbersome, decimal underscores the modern relationship between the metric system and customary units, anchoring the ounce to the international prototype of the kilogram and, since 2019, to the Planck constant via the redefined kilogram.

In practical application, this conversion's decimal nature means it is almost always rounded, with the degree of rounding dictated by the required precision. For nutritional labeling in the U.S., the Food and Drug Administration mandates that one ounce be rounded to 28 grams, a simplification for public understanding. In scientific or technical fields requiring high accuracy, the full conversion factor is used. The persistence of the ounce alongside the gram creates a landscape where dual-unit labeling is common, particularly in international trade and product manufacturing, requiring consistent application of the conversion to avoid costly errors in formulation, dosing, or shipping weight calculations.

Therefore, the direct answer is 28.349523125 grams per avoirdupois ounce. The more significant implication lies in the operational necessity of knowing which system of ounce is being referenced and the appropriate level of precision for the task. The coexistence of measurement systems necessitates this precise, if occasionally inconvenient, conversion, making it a fundamental constant in global commerce, manufacturing, and daily life wherever customary units persist alongside the metric standard.