Glossary · Fine Jewelry
Troy Ounce
A troy ounce is 31.1035 grams, roughly ten per cent heavier than the avoirdupois ounce used for groceries (28.35 g). Precious metals are quoted per troy ounce, so any gold spot price you read is a troy-ounce price. One troy ounce holds exactly twenty pennyweights. Confusing the two ounces misstates a metal value by about a tenth, which matters on every melt or scrap calculation.
| Unit | Grams | Used for |
|---|---|---|
| Troy ounce (ozt) | 31.1035 g | Gold, silver, platinum — every spot price |
| Avoirdupois ounce (oz) | 28.3495 g | Food, post, everything else |
| Pennyweight (dwt) | 1.5552 g | Bench work and scrap quotes |
| Gram (g) | 1 g | Retail listings, most of the world |
The ten per cent that catches people out
A troy ounce is about 9.7 per cent heavier than the ounce on a kitchen scale. Weigh a chain on a scale set to ordinary ounces, read the number as troy, and the metal appears worth nearly a tenth more than it is. The error runs the other way just as easily. If a scale offers both, check which one it is in before doing any arithmetic — the two figures look equally plausible and nothing on the display says which convention you are in.
Twelve to the pound, not sixteen
The troy system keeps its own pound as well: twelve troy ounces, not the sixteen of the avoirdupois pound. A troy pound is therefore lighter than a standard pound even though its ounces are heavier. The troy pound has almost no modern use — metals are quoted per ounce — but it explains why the two systems cannot be casually mixed.
Common questions
- Why is gold weighed in troy ounces?
- It is the surviving standard for precious metals, and every spot price you read is quoted in it. Using the everyday ounce instead would misstate the value by about a tenth.
- How many grams are in a troy ounce?
- 31.1035 grams, against 28.3495 for the avoirdupois ounce used for groceries.
- Is a troy ounce heavier than a regular ounce?
- Yes, by roughly 9.7 per cent. But a troy pound is lighter than a standard pound, because it holds twelve ounces rather than sixteen.
Related terms
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