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 an avordupois ounce) is used to weigh other commodities such as sugar and grain. A standard ounce is slightly less than a troy ounce, at 28.35 grams.

The troy ounce is to this day the standard unit of measurement in the precious metals market to ensure purity standards. The troy ounce is often abbreviated to read "t oz" or "oz t." 

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 The troy weight system, however, has largely been replaced by the carat system in the world of precious gems and stones.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 believed to have gotten its name from the French city of Troyes, a business hub in the Middle Ages 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, however, 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 their new monetary system on the weights developed by 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 better reflect 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. The word means "goods of weight" and was used to weigh both precious metals and non-precious metal goods. By 1527, the troy ounce became the official standard measurement for gold and silver in Britain, and the U.S. followed suit in 1828.

Troy Ounce vs. Ounce

The avoirdupois ounce, simply referred to as ounce (oz), is the standard used in the US to measure foods and other items, with the exception of precious metals and gems. 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 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	0.0685
Kilograms	0.0311
Grains	480
Pennyweight (dwt.)	20
Carats	155.517

Investing in Precious Metals

The difference between a troy ounce and an avoirdupois ounce is of concern mostly to investors in precious metals. Gold has historically been seen as the ultimate safe haven, although its market price is volatile, not least because of its status as an imperishable asset. But the age-old practice of using gold and silver as a store of wealth has been joined in modern times by two more precious metals:

• Platinum is a relative newcomer to international precious metals trading, having been "discovered" by Europeans in South America in the first half of the eighteenth century. This silvery-white metal is used in high-end jewelry but its main use is in catalytic converters for vehicles. It also has uses in the chemicals industry.
• Palladium is rarely found in nature but can be extracted for commercial use from other metals including platinum, copper, and zinc. It has the same industrial uses as platinum and also is used in medical, dental, and electronics equipment.

Ways to Invest in Precious Metals

Most modern investors probably don't want to buy precious metals by the troy ounce and stow them in a safe at home. Luckily, there are modern ways to invest in these metals:

• Commodity exchange-traded funds (ETFs) or mutual funds are the easy way to get into investing in precious metals. For example, you decide to invest in one platinum ETF. Just as with any fund, you don't take delivery of the metals you own through the fund, but you can buy and sell fund shares all the same.
• Common stock shares of companies in the metals and mining sector are a more direct way to gain a stake in precious metals. They're riskier, too. You're putting your money on a single company's luck at locating and extracting the precious stuff.
• Futures and options markets exist for precious metals. Internet access has made all things accessible but these markets are dominated by professional traders and institutional investors.
• Gold certificates, once a usable form of government-issued currency, can still be purchased from some banks and investment companies. They are basically a portable gold equivalent, but they're no longer backed by the full faith and credit of any government. They're backed by the entity they are purchased from, with the potential risk that implies.

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.

Correction—Nov. 5, 2022: A previous version of this article misstated the conversion amounts for a pound and a kilogram, as it relates to the troy ounce.