Have you heard a coin’s weight being given in “troy ounces” or “avoirdupois ounces” and wondered, “what’s the difference?!”
This word comes from the Old French “aveir de peis,” or “goods of weight.” For most any item other than precious metals, if someone lists a weight in ounces, it’s assumed that this is the measurement they mean. The most commonly held explanation for the origin of troy ounces comes not from ancient Greece, but instead from the city of Troyes, France sometime around the middle ages.
Turns out, there IS something in a name. Troy ounces are generally used when weighing precious metals (like in America’s popular Silver Eagle series). Things like groceries or every day items are weighed in avoirdupois ounces. For example, anytime you step on a scale you’re being weighed based on the avoirdupois system. We’ve used avoirdupois ounces as our every day weight system for so long, it’s not really necessary to specify which type of ounce we’re measuring in.
It’s like comparing apples and… gold.
You might be wondering – but isn’t an ounce an ounce? In short, no. An avoirdupois ounce weighs 28.3495 grams, while a troy ounce weighs 31.1035 grams. It’s believed that this is a throwback to the historic importance of precious metals in trade markets, and merchants’ need to have a standard unit of weight for gold, silver, gemstones and the like.
It might not seem like a great difference between the two types of ounces. However, it does become more noticeable when you start talking about larger amounts. Not to mention that that’s when the real fun starts – because even though a troy ounce is more than an avoirdupois ounce, a troy pound weighs LESS than an avoirdupois pound.
Wait – how’s that possible? It is because a troy pound is a specific unit, defined as 12 troy ounces. But under the metric system, a pound is 16 ounces. So if you measure pounds in grams:
- 1 troy pound = 373.242 grams
- 1 avoirdupois pound = 453.592 grams
It can be a hefty concept to grasp, I know! But it’s important information for anyone who buys precious metal. So the next time you see a gold, silver or copper piece whose weight is listed in ounces, be sure you know if you’re buying troy, or avoirdupois!