What is the difference between a troy ounce and a regular ounce?

Precious metals are always weighed in troy ounces. When dealing with those metals, “troy ounce” and “ounce” are interchangeable. Non-precious metals (e.g., copper) are weighed in regular ounces. A troy ounce weighs approximately 31.104 grams, while a regular, or “avoirdupois,” ounce weighs approximately 28 grams. A kilo bar of silver weighs 32.15 troy ounces, whereas a kilo bar of copper weighs 35.27 ounces.