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." 

Understanding 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.

Key Takeaways

  • The troy ounce is a metric used in weighing precious metals.
  • The troy ounce is the equivalent of 31.1034768 grams, whereas the ounce is the equivalent of 28.349 grams.
  • A troy pound (12 troy ounces) is lighter than a standard pound (14.6 troy ounces).
  • The troy ounce is the last remaining metric still used in the troy weighting system.

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 before 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. Before 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 US followed suit in 1828.

Troy Ounce vs. Ounce

The avoirdupois ounce, simply referred to as ounce (oz), is a metric commonly used in the US to measure foods and other items, except precious metals. It is the equivalent of 28.349 grams or 437.5 grains. A troy ounce is a little heavier, with a gram equivalent of 31.1. The difference (2.751) may be minute for a small quantity, but it can be substantial for large quantities.

When the price of gold is said to be US $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. This pound is not to be confused with a troy pound, which is lighter and is made up of 12 troy ounces.