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The Difference Between Troy Ounce and Avoirdupois Ounce

As you begin buying and selling Gold, Silver, and other Precious Metals, you will notice that Precious Metals are calculated in a different unit of measurement than everyday items like sugar or chocolate. These metals are weighed using the traditional troy weight system as opposed to the more familiar “avoirdupois” units that we employ when weighing other goods.

Why Are Troy Ounces Important?

If you were to take one-ounce Gold coin from your personal collection and place it on a standard grocery-store scale, you would find that your coin is approximately 10% heavier than an ounce of beef or mushrooms or chocolate. Simply put, the two measures of an ounce are not the same. Many people don’t completely understand the troy ounce system, which is how new Precious Metals buys fall victim to unscrupulous sellers who try to boost the value of their products by simply listing them improperly. For example, a seller could claim that they are offering a 100-ounce Precious Metal bar, but you would shorted by about 10% of its value because the bar actually only weighs 90 troy ounces.

This may seem complicated—unnecessarily complicated—but there are good reasons for Precious Metals maintain a distinct unit of measure. The troy ounce was retained as the standard unit of measurement even after the adoption of the avoirdupois ounce to ensure purity standards and other common Precious Metal measures remained consistent, worldwide and over time.

History of the Troy Ounce

There is some historical opacity as to the precise origins of Troy weight system, but it is commonly believed that the name is derived from Troyes, a trade market in France. Merchants converged on that area from all over the world to buy and sell goods, so a standardized unit of weight would have made doing business much more efficient. Later, many areas of Europe devised their own version of the troy ounce, but the French troy system is believed to be most closely related to the one that we still use today.

If you wish to trace the origins of the troy ounce back even further, you would need to delve into the Roman monetary system. Roman currency took the form of bronze bars, and the bar that would be equal to one pound today was referred to as an “aes grave.” A twelfth of that was called an “uncia,” or ounce. There are 12 troy ounces to a troy pound.

The Troy Ounce Today

Prior to the adoption of the metric system, French-born King Henry II of England chose to adjust the British coinage system to be more in line with the French troy system he felt was superior. The system was tinkered with periodically, but the troy weights we know today were first used in England during the 15th century. By 1527, the troy ounce had become the official standard measurement for Gold and Silver in Britain, with the U.S. finally adopting the troy system in 1828.

Just like the worldwide traders in Troyes, France, buyers and sellers today need a standardized form of measurement for Precious Metals.

Troy Ounces vs. Avoirdupois Ounces

So, how does an ounce of Gold differ from an ounce of chocolate? While Gold is measured by troy ounces, goods such as chocolate or sugar or coffee are measured using avoirdupois ounces. The name may sound odd to you unless you know French, but the word avoirdupois is literally translated as “goods of weight” and is simply the unit of measure we in the United States use to weigh almost everything other than Precious Metals. The scales in the produce department at your local grocery and your bathroom scales weigh things in avoirdupois ounces. Because the avoirdupois ounce is our standard system in the U.S, we simply refer to it as “ounces.”

Troy Ounce vs Regular Ounce

A troy ounce, then, is a bit heavier than a regular ounce. One regular ounce is 28.35 grams, while a troy ounce translates to 31.1 grams. These weights are so close as to seem almost negligible, but the difference becomes noticeable quickly when you’re trading large quantities of Precious Metals.

Here is the point where troy ounces become extremely fascinating and the most confusing. Remember our discussion about how the troy system was indirectly derived from the Roman monetary system, using 12 troy ounces to make up one troy pound? This means that a troy pound is lighter than a regular pound, even though troy ounces are heavier! This is only possible because  there are fewer of these larger ounces in a troy pound—12 as opposed to the 16 ounces for a regular pound.

Understanding Troy Ounces Online

The system may seem confusing if you aren’t familiar with it, but it does make sense in an historical context and you can definitely master it quickly if you wish to. At APMEX, we already list all of our Precious Metals by the troy system as all reputable Precious Metals retailers should. Of course, if you ever happen across a piece listed in avoirdupois ounces, you can easily convert it to troy ounces by multiplying the stated weight by .91. 

It is easy to see why a complete understanding of the troy system is key for Precious Metals investors and collectors. This will help you navigate the Precious Metals market secure in the knowledge that you can’t be confused or taken in by unscrupulous sellers.

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