Key points
- A troy ounce is 31.1035 grams – not 28.35 g like the “ordinary” ounce.
- What counts is the fine weight, not the total weight.
- Formula: weight × fineness ÷ 1000 × spot price per gram.
- For scrap gold, dealers usually pay less than the pure metal value.
The right ounce
Precious metals are weighed in troy ounces (oz). One troy ounce equals 31.1035 grams. The everyday ounce (used for food in the US, for example) is only 28.35 g. Mix them up and you will be off by almost 10 %.
- 1 oz = 31.1035 g
- 1 kg = 32.15 oz
- Spot price per gram = spot price per ounce ÷ 31.1035
Fineness and carats
Hardly any gold piece is 100 % gold. Fineness states in thousandths how much pure gold it contains. For jewellery, carats are common (24 carats = pure).
| Stamp | Carat | Gold content | Typical for |
|---|---|---|---|
| 999.9 | 24 | 99.99 % | Bars, Maple Leaf, Philharmonic |
| 986 | 23.6 | 98.6 % | Ducats |
| 916 | 22 | 91.6 % | Krugerrand, older Britannia, American Eagle |
| 900 | 21.6 | 90 % | Historic coins (Vreneli, Goldmark, pesos) |
| 750 | 18 | 75 % | Fine jewellery |
| 585 | 14 | 58.5 % | Most common jewellery in Germany |
| 333 | 8 | 33.3 % | Budget jewellery |
The formula
Metal value = weight (g) × fineness ÷ 1000 × spot price per gram
What matters is the fine weight: a Krugerrand weighs 33.93 g but, because of its copper alloy, contains exactly one troy ounce of gold. The rest just makes it more scratch-resistant.
Worked examples
Suppose gold costs €100 per gram (an example only, not a current price):
| Piece | Calculation | Metal value |
|---|---|---|
| Krugerrand 1 oz | 33.93 g × 0.9167 = 31.10 g | €3,110 |
| Mexican 50 pesos | 41.67 g × 0.900 = 37.50 g | €3,750 |
| Swiss Vreneli 20 francs | 6.45 g × 0.900 = 5.81 g | €581 |
| 585 chain, 20 g | 20 g × 0.585 = 11.70 g | €1,170 |
For jewellery, the metal value is an upper limit. Dealers and refiners deduct costs for melting and refining and therefore pay less. Designer or antique jewellery, on the other hand, can be worth more.
Common mistakes
- Using total weight instead of fine weight – with 585 jewellery an error of more than 40 %.
- Confusing ounces: a troy ounce is not an ordinary ounce.
- Weighing stones and clasps: gemstones, steel springs or pearls are not part of the gold weight.
- Using an old price: the spot price moves constantly. Calculate with the current value.
This guide is general information and does not replace investment, tax or legal advice. No guarantee of accuracy; rules can change. Updated: 30 September 2026.