The standard weight measurements used for all precious metal products are Troy ounces and pounds. All legal tender Silver, gold and Platinum coins are struck according to these Troy weights, which are not to be confused with the Avoirdupois weights. Avoirdupois measurements are used for coarse dry weights such as grain, flour, sugar, tea, etc., and are the units of weight you likely learned about in school.
The term Avoirdupois was adapted from the French phrase “aveir de pois” or “aver de peis” – which roughly translates to “goods of weight.” Precious metals, gems and drugs use the Troy system. By the way, the Troy system is not named for the Ancient Greek City of Troy, but for Troyes, France – where they were first introduced. Troy weights are also called Apothecary weights, due to their use with medical drugs.
Here are the basic specification comparisons between Troy and Avoirdupois weights:
Troy Ounces > Avoirdupois Ounces
1 Troy Ounce = 31.1035 grams
1 Avoirdupois Ounce = 28.3495 grams
In other words, a 1 ounce silver coin is heavier than an ounce of tea.