Why Gold Is Weighed and Sold in Ounces: The History of the Troy Ounce
Prices for investment gold and silver are quoted per troy ounce all over the world. Why the ounce, where did this unit come from and how much does it actually weigh? Let’s look at its history and what it means in practice for investors.
What is a troy ounce
The troy ounce (abbreviated oz t or simply oz) is a unit of weight used exclusively for precious metals. It weighs 31.1035 grams, usually rounded to 31.1 g in practice. It is the unit in which the world price of gold, silver, platinum and palladium is quoted.
Where the name “troy” comes from
The most widespread explanation links the unit to the French town of Troyes, an important trading centre in the Middle Ages with famous fairs. Merchants from all over Europe needed a common system of weights for precious metals and coins there – and the troy system gradually spread and has survived to this day. The name has nothing to do with ancient Troy.
Troy ounce vs. ordinary ounce
Beware of confusing the two: a troy ounce is not the same as the ordinary (avoirdupois) ounce you may know from the kitchen.
- Troy ounce: 31.1035 g (precious metals),
- Ordinary ounce: 28.3495 g (food, everyday goods).
The troy ounce is therefore slightly heavier. Another oddity is the troy pound – it has only 12 troy ounces (373.24 g), whereas an ordinary pound has 16 ounces.
Why precious metals are still traded in ounces
The troy system became the international standard for precious metals and has stayed that way. The global wholesale price (the spot price) is set per troy ounce, and prices around the world are derived from it. A single unit makes trade across countries and currencies clear and comparable – which is why the best-known bullion coins weigh exactly 1 troy ounce.
Ounces or grams? Both, in practice
Investment gold coins are traditionally struck in ounces and fractions of an ounce (1 oz, 1/2 oz, 1/4 oz, 1/10 oz). Investment bars, on the other hand, are usually made in grams in Europe (1 g, 5 g, 10 g, 100 g …). It is therefore useful for investors to be able to convert between the units.
Quick conversion
- 1 oz = 31.1035 g,
- 1/2 oz = 15.55 g,
- 1/4 oz = 7.78 g,
- 1/10 oz = 3.11 g,
- 1 g = 0.0322 oz.
What it means for investors
When comparing offers, always look at the price per gram of pure metal – that way you can compare a coin in ounces with a bar in grams. The premium over the metal price also plays a role and is usually higher for smaller weights. This is explained in detail in the article Premium on gold: why the price per gram differs. The same units apply to investment silver.
Frequently asked questions
How much does a troy ounce of gold weigh?
31.1035 grams; in practice 31.1 g is quoted.
Is a troy ounce the same as an ordinary ounce?
No. The troy ounce (31.1 g) is heavier than the ordinary ounce (28.35 g) and is used only for precious metals.
Why is the gold price quoted per ounce rather than per gram?
It is a long-established international standard. The spot price is quoted per troy ounce, and prices around the world are derived from it.
Conclusion
The troy ounce is a legacy of medieval trade that still accompanies precious metals today. For investors, the key is being able to convert ounces to grams and compare the price per gram. Browse our investment gold and gold coins.
About the author
Adolf Baumann, analyst at Baumann Group a.s. (GOLDU), has been working with precious metals for over 30 years. He has seen euphoria, crashes and panic on the markets – and he still enjoys it. He explains in plain language what moves the price of gold and silver and how to handle them without unnecessary mistakes. His weakness? He refuses to wear a suit.
