A troy ounce is the unit used for precious metals and equals exactly 31.1034768 grams. It is about 10% heavier than the everyday (avoirdupois) ounce of 28.35 g that you see on food labels. Gold is priced worldwide per troy ounce for historical reasons, so the gold price quoted on the news is almost always a price for one troy ounce.
How much a troy ounce weighs
When people talk about gold, “ounce” almost always means troy ounce. The key figures are:
| Measure | Value |
|---|---|
| 1 troy ounce | 31.1034768 g |
| 1 avoirdupois ounce | 28.3495 g |
| 1 kilogram | 32.1507 troy ounces |
| 1 troy pound (12 troy ounces) | 373.24 g |
| 1 pennyweight (dwt, 1/20 troy oz) | 1.555 g |
The difference between the two ounces is about 2.75 g, or nearly 10%. Mix them up and any valuation will be off by the same amount.
A curiosity: the troy pound has 12 ounces rather than 16, so a troy pound is lighter than an ordinary pound even though a troy ounce is heavier than an ordinary ounce.
Where the troy ounce comes from
The name is usually linked to Troyes, a French market town whose medieval fairs drew merchants from across Europe who needed a common system of weights. The origin is not certain, but this is the most widely accepted explanation.
England made the troy pound the basis for gold and silver in 1527 under Henry VIII. The United States followed: an act of Congress in 1828 adopted the troy pound as the standard for US coinage. London and New York grew into the two largest gold markets and both kept the unit, which is why it is still the global standard for precious metals.
Why gold is still weighed in troy ounces
The reasons are market habit and the need for a single standard:
- Benchmark prices are per troy ounce. The London market, overseen by the LBMA, sets gold and silver prices in US dollars per troy ounce, and US spot prices are quoted the same way.
- Futures contracts are in troy ounces. The standard COMEX gold futures contract covers 100 troy ounces.
- Large bars are measured in troy ounces. A London Good Delivery gold bar must contain between 350 and 430 fine troy ounces, typically around 400 oz, or about 12.4 kg.
- Bullion coins come in ounce sizes. The American Eagle, American Buffalo, Maple Leaf and Krugerrand are sold in 1 oz and fractions of it.
In the US, most retail bars are sold in ounces too (1 oz, 10 oz, 100 oz, plus kilo bars), although gram bars are also available. Jewelers and scrap buyers often weigh in pennyweights.
Converting troy ounces and grams
The formulas are simple:
- grams = troy ounces × 31.1035
- troy ounces = grams ÷ 31.1035
- price per gram = price per troy ounce ÷ 31.1035
Handy values for common sizes:
| Size | Grams | Troy ounces |
|---|---|---|
| 1/10 oz | 3.11 g | 0.1 |
| 1/4 oz | 7.78 g | 0.25 |
| 1/2 oz | 15.55 g | 0.5 |
| 1 oz | 31.10 g | 1 |
| 10 oz bar | 311.03 g | 10 |
| 100 g bar | 100 g | 3.215 |
| 1 kg bar | 1,000 g | 32.15 |
Worked example: at an illustrative price of $1,000 per troy ounce, one gram would cost $1,000 ÷ 31.1035 = about $32.15, and one pennyweight about $50. For today’s figures see live US gold prices, or let our gold calculator do the math for any weight or karat.
Gross weight versus fine gold content
When a coin is described as “1 oz,” that refers to the pure gold it contains, not necessarily what it weighs on a scale:
- American Gold Buffalo 1 oz – .9999 fine; virtually its whole weight is gold.
- American Gold Eagle 1 oz – 22K, weighs 33.931 g (1.0909 troy oz) but contains exactly one troy ounce of gold.
- Krugerrand 1 oz – 22K, weighs 33.93 g, one troy ounce of gold.
- Pre-1933 $20 Double Eagle – .900 fine, contains about 0.9675 troy ounces of gold.
When comparing prices, look at how much pure gold you are getting rather than the coin’s total weight.
Reading a spot price quote
A spot quote such as “gold $X/oz” is the wholesale price for one troy ounce of pure gold for immediate delivery, usually with a bid (what buyers pay) and an ask (what sellers want). Retail products cost more than spot because of fabrication, distribution and dealer costs, and dealers buy back below their selling price. Silver quotes work the same way, also per troy ounce, and so do platinum and palladium.
Common mistakes with ounces
- Using a kitchen scale or general converter. Both use the avoirdupois ounce, which understates the amount of gold by about 10%.
- Comparing spot with a dealer’s price directly. Dealer prices include a premium over spot, which differs by product.
- Rounding too early. On a 1 kg bar, using 31.1 instead of 31.1035 g per ounce shifts the result by about 0.1 oz.
- Confusing pennyweights with grams. A scrap quote per dwt is about 1.555 times the price per gram.
- Assuming silver uses a different ounce. It does not: silver, platinum and palladium are also weighed in troy ounces of 31.1035 g.
You can follow how the per-ounce price has moved on our gold price chart.
Bottom line
A troy ounce is 31.1034768 g, about 10% heavier than an ordinary ounce. Global gold and silver prices, futures and large bars are all measured in troy ounces. Divide the per-ounce price by 31.1035 to get the price per gram; a coin’s ounce size refers to its pure gold content.
Sources: LBMA – The Troy Ounce · NIST – SI units of mass · LBMA – London Good Delivery
This is general information, not tax or investment advice.
Frequently asked questions
How many grams are in a troy ounce?
Exactly 31.1034768 grams, usually rounded to 31.1 g.
Is a troy ounce heavier than a regular ounce?
Yes. A regular (avoirdupois) ounce is 28.35 g, while a troy ounce is 31.10 g, about 10% heavier.
How many troy ounces are in a kilogram?
About 32.15 troy ounces.
What is a pennyweight?
A pennyweight (dwt) is 1/20 of a troy ounce, or about 1.555 g. US jewelers and scrap buyers often use it.
Why does a 1 oz Gold Eagle weigh more than an ounce?
It is 22K gold, so it includes copper and silver alloy. It weighs 33.931 g but contains exactly one troy ounce of pure gold.
