What Is a Troy Ounce?
A troy ounce is the standard unit of weight for precious metals. It equals 31.1035 grams. When someone says “gold is $4,800 per ounce,” they mean per troy ounce. Every spot price quote, every coin weight specification, and every bar marked “1 oz” uses the troy system.
The standard ounce used for everything else (food, postage, body weight) is the avoirdupois ounce, weighing 28.3495 grams. A troy ounce is approximately 9.7% heavier than a standard ounce.
This difference matters. If you weigh a 1 oz gold coin on a kitchen scale set to avoirdupois ounces, it will read approximately 1.097 oz. That is correct. The coin contains 1 troy ounce (31.1g), which equals 1.097 avoirdupois ounces.
Why Do Precious Metals Use Troy Weight?
The troy system originated in Troyes, France, a major medieval trading hub where merchants from across Europe gathered at seasonal fairs. By the 12th century, the troy weight system had become the standard for precious metals transactions throughout Europe.
England adopted troy weight for its coinage in the 15th century. The British Royal Mint has used troy ounces since at least 1527. When the United States established its coinage system in 1792, it inherited the troy standard from British practice. The US Mint has measured gold and silver in troy ounces for over 230 years.
The system persists because global precious metals markets need a single, universally understood unit of measurement. Changing to metric grams would be technically simple but practically disruptive, requiring every futures contract, spot price quote, and coin specification worldwide to be restated. The London Bullion Market Association (LBMA), COMEX futures exchange, and every major mint all use troy ounces. That consensus is self-reinforcing.
How Do Troy and Avoirdupois Systems Compare?
The two systems diverge in counterintuitive ways, particularly when it comes to pounds.
Quick reference table:
| Measurement | Troy System | Avoirdupois System |
|---|---|---|
| 1 ounce | 31.1035 grams | 28.3495 grams |
| 1 pound | 12 ounces (373.24g) | 16 ounces (453.59g) |
| Ounces per pound | 12 | 16 |
| Grams per ounce | 31.1035 | 28.3495 |
The confusing part: a troy ounce is heavier than an avoirdupois ounce, but a troy pound is lighter than an avoirdupois pound. This is because a troy pound contains only 12 troy ounces (373.24g), while an avoirdupois pound contains 16 avoirdupois ounces (453.59g).
In practice, troy pounds are almost never used in modern commerce. Precious metals are quoted in troy ounces, grams, or kilograms. The troy pound survives mainly as a historical curiosity and occasional source of confusion.
What Are the Conversion Formulas?
Troy ounces to grams: Troy ounces x 31.1035 = grams
Grams to troy ounces: Grams / 31.1035 = troy ounces
Troy ounces to avoirdupois ounces: Troy ounces x 1.09714 = avoirdupois ounces
Avoirdupois ounces to troy ounces: Avoirdupois ounces x 0.91146 = troy ounces
Troy ounces to kilograms: Troy ounces x 0.0311035 = kilograms
Kilograms to troy ounces: Kilograms x 32.1507 = troy ounces
Common conversions for precious metals:
| Amount | Troy Ounces | Grams | Kilograms |
|---|---|---|---|
| 1/10 oz coin | 0.1 | 3.11 | 0.00311 |
| 1/4 oz coin | 0.25 | 7.78 | 0.00778 |
| 1/2 oz coin | 0.5 | 15.55 | 0.01555 |
| 1 oz coin/bar | 1 | 31.10 | 0.03110 |
| 5 oz bar | 5 | 155.52 | 0.15552 |
| 10 oz bar | 10 | 311.03 | 0.31103 |
| 1 kilo bar | 32.1507 | 1,000.00 | 1.00000 |
| 100 oz bar | 100 | 3,110.35 | 3.11035 |
| 400 oz bar (LBMA Good Delivery) | ~400 | ~12,441 | ~12.44 |
How Does This Affect Coin Weights?
Not all “1 oz” coins weigh exactly 31.1 grams on a scale. Some coins contain 1 troy ounce of pure gold but weigh more due to alloying metals.
American Gold Eagle: Contains exactly 1 troy ounce (31.1g) of pure gold, but the total coin weight is 33.93 grams. The additional 2.83 grams come from copper and silver alloy that gives the coin its durability and distinctive color. The coin is .9167 fine (22 karat).
Canadian Gold Maple Leaf: Contains 1 troy ounce (31.1g) of pure gold, and the total coin weight is also 31.1 grams. The coin is .9999 fine (24 karat), meaning it is essentially pure gold with no alloying metals.
South African Krugerrand: Contains 1 troy ounce (31.1g) of pure gold, total weight 33.93 grams. Like the Eagle, it uses copper alloy (.9167 fine), giving it a slightly reddish hue.
When verifying authenticity, weigh the coin and compare to published specifications for that specific product. A 1 oz Gold Eagle should weigh 33.93g total. A 1 oz Maple Leaf should weigh 31.10g total. Deviations of more than 0.1g warrant further investigation.
Frequently Asked Questions
Why not just use grams?
The international gold market quotes prices in both troy ounces and grams, but troy ounces remain the primary unit for spot pricing, futures contracts, and coin specifications in Western markets. Some countries, particularly in Asia, commonly quote gold in grams or taels (a Chinese unit equal to approximately 37.5 grams or 1.2 troy ounces). The LBMA and COMEX, which set global benchmark prices, use troy ounces.
Is a “gold ounce” always a troy ounce?
Yes. In the context of precious metals, “ounce” always means troy ounce unless explicitly stated otherwise. This is universal across dealers, exchanges, mints, and pricing services. If a dealer advertises “1 oz gold bar,” it contains 31.1 grams of gold.
How do I verify a coin’s weight at home?
A digital scale accurate to 0.01 grams costs $15 to $30 and is the most practical authentication tool for home use. Weigh the coin and compare to the published weight for that specific product. Weight alone does not prove authenticity (counterfeits can match weight using tungsten cores), but an incorrect weight immediately identifies a fake. For additional verification, measure dimensions with calipers and test specific gravity using the water displacement method.
Do silver bars and coins also use troy ounces?
Yes. All precious metals (gold, silver, platinum, palladium, rhodium) use the troy ounce as the standard unit. A “10 oz silver bar” contains 10 troy ounces (311.03 grams). Silver spot prices are quoted per troy ounce.
What is a tael?
A tael (also called a tola in South Asia) is a traditional unit of weight used in Chinese, Hong Kong, and other Asian gold markets. One Hong Kong tael equals 37.429 grams, or approximately 1.2034 troy ounces. Gold bars denominated in taels are common in Asian markets but uncommon in Western retail. If purchasing tael-denominated bars, convert to troy ounces for price comparison purposes.