Troy Ounce: History and Conversion

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 (also known as the Avordupois ounce) is instead used to weigh other commodities such as sugar and grain, and 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." 

Key Takeaways

  • The troy ounce is a metric used in weighing precious metals.
  • The troy ounce is the equivalent of 31.1034768 grams.
  • A standard ounce is the equivalent of 28.349 grams, or around 10% less.
  • 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.

Understanding the Troy Ounce

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. Thus, when prices of precious metals are listed per-ounce, it is often referencing the troy ounce.

It was also used when weighing certain gemstones. The troy weight system, however, has largely been replaced by the carat system in the world of precious gems and stones.

History of the Troy Ounce

The troy system for weights and measures is thought to originate from the middle-ages French city of Troyes, a mercantile hub that attracted merchants from around Europe and Britain. In Troyes, merchants measured 480 grains of barley to equal a troy ounce, and 12 troy ounces equaled one troy pound. 

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.

Before the adoption of the metric system across Europe, the 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 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.

Troy Ounce Conversion

Below is a quick reference table for converting troy ounces into other common weights.

Troy Ounce Conversion Table
1 Troy Ounce =
 Standard Ounce  1.097
 Grams 31.103
Pounds 0.686
Kilograms 0.311
Grains 480
Pennyweight (dwt.) 20
Carats 155.517

Which Is Heavier: 1 oz. or 1 Troy Ounce?

A troy ounce is approximately 10% heavier than a standard ounce (1.0:1.097).

What Are Troy Ounces Used for Today?

Troy ounces are still used in measurements of precious metals (like gold and silver) and gemstones. Most other objects use either the metric system and/or standard ounces.

What Is a Troy Pound?

A troy pound is 12 troy ounces (while a standard pound is 16 standard ounces). 1 troy pound = 0.823 troy pounds.

The Bottom Line

Troy ounces are used to measure weights of gems and precious metals. One troy ounce is equal to 1.097 standard ounces, making it around 10% heavier. The name dates back to the 16th century French town of Troyes, which was an important trade center that standardized the troy ounce as 480 grains of barley.

Article Sources
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  1. The Royal Mint. "What Is a Troy Ounce?"

  2. Karat Gold. "Weights and measures." Maguire Reference, 1982.

  3. JM Bullion. "What Is a Troy Ounce?"

  4. International Society of Weighing and Measurement. "Weights and Measures Tables."

  5. Metric Conversions. "Troy Pounds to Pounds."

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