What Is a Troy Ounce?
What is a troy ounce?
The troy ounce belongs to the troy weight system, an old family of units still used for precious metals. NIST, the U.S. agency for weights and measures, defines it as exactly 480 grains, or 31.1034768 grams. Most people round that to 31.1035 grams, or simply 31.1 grams.
Every precious metal price you see is quoted per troy ounce. Futures exchanges, the London market, the U.S. Mint and coin dealers all use it. The abbreviations you will see are “oz t,” “ozt” and sometimes just “oz.”
How is a troy ounce different from a regular ounce?
The ounce used for food, mail and body weight is the avoirdupois ounce. It weighs 28.3495 grams, or 437.5 grains. The troy ounce is heavier: 480 grains versus 437.5 grains.
That gap matters. If you put a one-ounce gold coin on a kitchen scale, it should read about 1.1 ounces, not 1.0. The scale is not wrong. It is simply counting in the smaller ounce.
Here is the twist that confuses people. The troy pound is lighter than the regular pound. A troy pound holds only 12 troy ounces (373.24 grams). A regular pound holds 16 avoirdupois ounces (453.59 grams). So a troy ounce is heavier, but a troy pound is lighter.
What are grains and pennyweights?
The troy system is built on the grain, the smallest unit. The grain is the same in troy and avoirdupois weight, which is what lets the two systems be compared.
- Grain (gr): 0.06479891 grams, exactly.
- Pennyweight (dwt): 24 grains, or about 1.5552 grams. Twenty pennyweights make one troy ounce. Jewelers and scrap buyers still use it.
- Troy ounce (oz t): 480 grains, or 20 pennyweights.
- Troy pound (lb t): 12 troy ounces, or 5,760 grains. It is rarely used today.
Troy weight conversion table
| Unit | Grams | Grains | Troy ounces |
|---|---|---|---|
| 1 grain | 0.0648 | 1 | 0.00208 |
| 1 gram | 1 | 15.432 | 0.03215 |
| 1 pennyweight | 1.5552 | 24 | 0.05 |
| 1 avoirdupois ounce | 28.3495 | 437.5 | 0.9115 |
| 1 troy ounce | 31.1035 | 480 | 1 |
| 1 troy pound | 373.2417 | 5,760 | 12 |
| 1 avoirdupois pound | 453.5924 | 7,000 | 14.583 |
| 1 kilogram | 1,000 | 15,432.36 | 32.1507 |
Two quick rules of thumb: one troy ounce is about 1.097 regular ounces, and one kilogram is about 32.15 troy ounces. For other amounts, use our weight converter.
Why do precious metals use troy ounces?
Mostly tradition. Troy weight was used to weigh precious metals in England for centuries, and the habit carried into modern markets. The name is often traced to the medieval trade fairs of Troyes, France. The London Bullion Market Association notes that this origin is likely but not proven.
Today the reason is consistency. Large wholesale contracts are written in troy ounces. A standard COMEX gold futures contract covers 100 troy ounces, and a silver contract covers 5,000 troy ounces. Because the whole market speaks one unit, a price in New York, London or a local coin shop refers to the same amount of metal.
How does this apply to coins?
When a modern bullion coin is called “one ounce,” it means it contains one troy ounce of the metal. The coin itself can weigh more. The one-ounce American Gold Eagle weighs 33.931 grams because it is 22-karat gold, with a little silver and copper mixed in. Its pure gold content is still one troy ounce. Terms like fineness and fine weight are defined in our glossary.
Older U.S. gold and silver coins were not made to round troy weights. Their metal content comes out to odd figures, such as 0.9675 troy ounces of gold in a pre-1933 $20 gold piece.
If you have a question about the metal content of a certified coin, the team at Affinity Collectibles in Ronkonkoma, NY is glad to help through our contact page.
Frequently asked questions
How many grams are in a troy ounce?
31.1034768 grams, usually rounded to 31.1035 or 31.1 grams.
Is a troy ounce heavier than a regular ounce?
Yes. A troy ounce is about 9.7% heavier than an avoirdupois ounce: 480 grains versus 437.5 grains.
How many troy ounces are in a kilogram?
About 32.15 troy ounces.
Why does my one-ounce gold coin weigh more than an ounce?
Two reasons. A kitchen scale reads in avoirdupois ounces, which are smaller. And many gold coins are alloyed, so the total weight is higher than the pure gold content stamped on the coin.
Sources
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement
- NIST: Precious Metals Conversion Information
- London Bullion Market Association: The Troy Ounce
- CME Group: COMEX Gold Futures rulebook (Chapter 113)
- CME Group: COMEX Silver Futures rulebook (Chapter 112)
- 31 U.S. Code § 5112 (American Eagle gold coin specifications)