Troy Ounce vs Avoirdupois Ounce — Gold, Silver, and Precious Metals Guide
If you have ever noticed that a price quote for gold says "per troy ounce" and wondered whether that is the same as a regular ounce — it is not. A troy ounce is heavier than the standard ounce used for food, body weight, and postal items. Confusing the two when buying or calculating the value of precious metals is a real and surprisingly common mistake.
The Weight Converter handles grams, ounces (avoirdupois), and kilograms. For troy ounces, this article explains the conversion and provides the reference table you need.
The Two Ounces
Troy ounce = 31.1035 grams
Avoirdupois ounce (standard ounce) = 28.3495 grams
A troy ounce is about 9.7% heavier than an avoirdupois ounce. The difference is 2.754 grams — not much on a single ounce, but meaningful when you are pricing gold.
| Unit | Grams |
|---|---|
| Troy ounce (t oz) | 31.1035 g |
| Avoirdupois ounce (oz) | 28.3495 g |
| Difference | 2.754 g |
Conversion formula:
troy ounces = avoirdupois ounces × 0.9115
avoirdupois ounces = troy ounces × 1.0971
grams = troy ounces × 31.1035
troy ounces = grams ÷ 31.1035
Where Each Ounce Is Used
Troy ounce: Used exclusively for precious metals — gold, silver, platinum, palladium. All spot prices you see quoted for gold ("gold price per ounce") are in troy ounces. The troy system is used globally for precious metals trading.
Avoirdupois ounce: Used for everything else — food, body weight, postal items, packaged goods, cooking. When someone says "1 ounce of chicken" or "the parcel weighs 3 ounces," they mean avoirdupois ounces.
The troy system is older. It was used in medieval Europe for gold and silver trade, with origins tracing to the market town of Troyes in France. The avoirdupois system developed for bulk goods — wool, grain — where a different pound and ounce structure was more practical.
Gold and Silver Price Calculation
When you see a gold price of, say, $2,000 per ounce, that is $2,000 per troy ounce. To find the price per gram:
price per gram = price per troy ounce ÷ 31.1035
At $2,000/troy oz: $2,000 ÷ 31.1035 = $64.31 per gram
To find the price per avoirdupois ounce (useful for comparison or if you have an item weighed in standard ounces):
price per avoirdupois oz = price per troy oz × 0.9115
At $2,000/troy oz: $2,000 × 0.9115 = $1,823 per avoirdupois oz
If you are selling a piece of jewelry that weighs 2 regular ounces (56.7 grams), the gold content calculation in troy ounces:
56.7 grams ÷ 31.1035 = 1.823 troy ounces
At $2,000/troy oz gold price: 1.823 × $2,000 = $3,646 worth of gold (if the item were pure gold).
Troy Weight System: Pounds and Pennyweights
The troy system has its own pound and smaller unit:
- Troy pound = 12 troy ounces = 373.24 grams
- Pennyweight (dwt) = 1/20 troy ounce = 1.5552 grams
- Grain = 1/480 troy ounce = 0.0648 grams (also used in the avoirdupois system, same size)
The troy pound is lighter than the avoirdupois pound (373.24 g vs 453.59 g). This is counterintuitive — a troy pound is lighter than a standard pound even though a troy ounce is heavier than a standard ounce. The reason: a troy pound has 12 ounces while an avoirdupois pound has 16, and the per-ounce difference does not offset the different ounce counts.
The troy pound is rarely used in practice. All precious metals pricing uses troy ounces.
Pennyweights come up in jewelry — a jeweler quoting a price "per pennyweight" means per 1/20 troy oz (1.555 g). If you see "dwt," that is pennyweights.
Reference Table: Troy Ounces to Grams
| Troy ounces | Grams | Kilograms |
|---|---|---|
| 0.1 t oz | 3.11 g | 0.00311 kg |
| 0.5 t oz | 15.55 g | 0.01552 kg |
| 1 t oz | 31.10 g | 0.03110 kg |
| 2 t oz | 62.21 g | 0.06221 kg |
| 5 t oz | 155.52 g | 0.15552 kg |
| 10 t oz | 311.03 g | 0.31103 kg |
| 20 t oz | 622.07 g | 0.62207 kg |
| 32.15 t oz | 1,000 g | 1 kg |
| 100 t oz | 3,110.35 g | 3.11035 kg |
Note: 1 kilogram = 32.1507 troy ounces (approximately 32.15). This is a useful number for precious metals — if you are looking at a 1 kg gold bar, that is about 32.15 troy ounces.
Common Precious Metal Bar Weights
Precious metals come in standard sizes, which mix the two systems depending on origin:
| Bar size | Troy ounces | Grams | Context |
|---|---|---|---|
| 1 gram bar | 0.032 t oz | 1 g | Small fractional gold |
| 1/10 oz | 0.1 t oz | 3.11 g | Small coin weight |
| 1 oz bar | 1 t oz | 31.10 g | Standard retail bar |
| 10 oz bar | 10 t oz | 311 g | Mid-size investment bar |
| 100 oz bar | 100 t oz | 3,110 g | Industrial/wholesale bar |
| 400 oz bar | 400 t oz | 12,441 g | London Good Delivery bar |
| 1 kg bar | 32.15 t oz | 1,000 g | Common in Asia and Europe |
The London Good Delivery bar — the standard for wholesale gold trading — is approximately 400 troy ounces, though bars between 350 and 430 troy ounces qualify. These are the large bars you see in vault images.
Jewelry: Carats and Purity vs Weight
When dealing with gold jewelry, "carat" refers to purity, not weight:
- 24 carat (24K): 100% pure gold
- 18 carat (18K): 75% gold (18/24)
- 14 carat (14K): 58.3% gold
- 9 carat (9K): 37.5% gold (common in UK jewelry)
To find the gold content of a piece:
gold weight = total weight × (carat ÷ 24)
A 10-gram, 18K gold ring contains: 10 g × (18/24) = 10 × 0.75 = 7.5 grams of pure gold
In troy ounces: 7.5 g ÷ 31.1035 = 0.241 troy ounces
At $2,000/troy oz gold price: 0.241 × $2,000 = $482 in gold content
The Weight Converter handles grams, standard ounces, and kilograms. For troy ounce conversions specifically, the formula is grams ÷ 31.1035.
