What Is a Troy Ounce?

A troy ounce is a unit of measure used for weighing precious metals that dates back to the Middle Ages. Originally used in Troyes, France, one troy ounce is equal to 31.1034768 grams, according to the U.K. Royal Mint. One standard ounce, used to weigh other items such as sugar and grain, is slightly less at 28.35 grams. The troy ounce is retained even today as the standard unit of measurement in the precious metals market to ensure purity standards and other common measures remain consistent over time. The troy ounce is often abbreviated to read "t oz" or "oz t." 

History of the Troy Ounce

Some historians believe the troy ounce had its origins in Roman times. Romans standardized their monetary system using bronze bars that could be broken down into 12 pieces called "uncia" or ounce, with each piece weighing around 31.1 grams. As Europe's economic importance grew from the 10th century onward, merchants came from all over the world to buy and sell goods there. It was therefore necessary to develop a new standardized monetary weight system to make doing business much easier. Some believe the merchants of Troyes modeled this new monetary system using the same weights as their Roman ancestors.

The troy ounce is the only measure of the troy weighting system that is still used in modern times. It is used in the pricing of metals such as gold, platinum, and silver.

JM Bullion says that prior to the adoption of the metric system in Europe, French-born King Henry II of England adjusted the British coinage system to be more reflective of the French troy system. The system was adjusted periodically, but troy weights as we know them today were first used in England in the 15th century. Prior to the adoption of the troy system, the British used an Anglo-Norman French system called the avoirdupois system, which means "goods of weight" and was also used to weigh both precious metal and non-precious metal items. By 1527, the troy ounce became the official standard measurement for gold and silver in Britain, and the U.S. finally followed suit in 1828.

When the price of gold is said to be U.S. $653/ounce, the ounce being referred to is a troy ounce, not a standard ounce. Because a troy ounce is heavier than a standard ounce, there are 14.6 troy ounces — compared to 16 standard ounces — in one pound.