Two ounces, two measuring systems
The difference comes from two old systems of weight that survived for different kinds of trade. The ordinary ounce belongs to the avoirdupois system, used for most everyday goods. Precious metals are commonly weighed in the troy system, whose ounce is larger.
The systems divide a pound differently: an avoirdupois pound contains 16 ounces, while a troy pound contains 12. But the troy pound is not simply another name for the same-sized pound; its underlying units are arranged differently. Comparing the systems directly, a troy ounce equals 480 grains, while an ordinary ounce equals 437.5 grains. A grain is a small unit of mass, not a literal grain of food.
Why the distinction matters in a sale
A scale measures mass; the unit setting tells you which rulebook to use. When a precious-metal price is quoted per troy ounce, weighing the item in ordinary ounces and treating the numbers as interchangeable would misstate how much metal is present—and therefore its value at that price.
The useful check is the unit printed on the scale, label, or price quote. “Ounce” alone is not always enough: in bullion trading, the extra mass is built into the meaning of the word.
