Troy ounce, gram and carat: the units side by side
The world market quotes in troy ounces, your scales show grams and your ring carries a carat figure. Three conversions close that gap for you.
What exactly is a troy ounce?
A troy ounce is 31.1035 grams. It is an old unit of weight from the trade in precious metal, named after the French city of Troyes, and it is not the same as the ordinary ounce of 28.35 grams you see on a packet of coffee.
All international quotations – the LBMA fixing, the exchange prices, every rate chart – are stated per troy ounce. If you want to know what that is per gram, you divide by 31.1035.
A worked example. With gold at 3,600 dollars per troy ounce and the euro rate at 1.08 dollars per euro, that is 3,333 euros per ounce, or 3,333 ÷ 31.1035 = 107.17 euros per gram of fine gold.
From carat to pure gold
Carat is a ratio out of 24. Divide the carat figure by 24 and you have the share of pure gold: 14 carat is 14 ÷ 24 = 0.583, which corresponds to the stamp 585.
Your piece of jewellery weighs gross, but you are paid on the pure gold in it. If a 14-carat necklace weighs 20 grams, it contains 20 × 0.585 = 11.7 grams of fine gold.
- 24 carat – 999/1000 – factor 0.999
- 22 carat – 916/1000 – factor 0.916
- 18 carat – 750/1000 – factor 0.750
- 14 carat – 585/1000 – factor 0.585
- 9 carat – 375/1000 – factor 0.375
- 8 carat – 333/1000 – factor 0.333
The whole sum in a single line
Proceeds = gross weight × fineness × price per gram of fine gold. That is all there is to it. Anyone who names you a price without your seeing those three figures is giving you an outcome, not a calculation.
For the example above: 20 grams × 0.585 × 107.17 = 1,253.90 euros of pure gold value. What a buyer pays out of that depends on his margin – and he ought to name it.
Silver works out the same way, in different orders of magnitude
The same formula applies to silver, only the amounts lie a factor of eighty to a hundred lower. Sterling silver (925) weighing 500 grams contains 462.5 grams of fine silver; at a silver price of 1.60 euros per gram that is 740 euros.
With silver in particular it pays to check the sum yourself: the weight mounts up quickly, and half a per cent difference in fineness already amounts to tens of euros with a complete canteen of cutlery.
- Krugerrand, Maple Leaf or Philharmonic: which one sells fastest?All three contain exactly one troy ounce of fine gold. What you get for them still differs – that lies in how well known they are, the fineness and how easily they scratch.Read on
- How does a buyer check whether gold is real?Magnet, acid test, specific gravity and X-ray. What each method does and does not prove – and why a stamp alone is never enough.Read on
- Selling a diamond without a certificate: what does it cost you?Without a GIA or HRD report a buyer has to judge for himself, and uncertainty translates into a lower offer. How much that differs, and when applying for a certificate pays off.Read on
Questions about troy ounce, gram and carat: the units side by side
How many grams is a troy ounce?
31.1035 grams. It is not to be confused with the ordinary ounce of 28.3495 grams, which is the measure you see on a packet of coffee. All international quotations for precious metal are stated per troy ounce, so this is the figure to divide by.
Why does nobody simply calculate in kilos?
Tradition, and international agreement. All the major exchanges and the LBMA quote per troy ounce, and switching over would make every historical price series incomparable. So the troy ounce stays, and the conversion to grams is left to whoever needs it.
Is 585 the same as 14 carat?
Yes. 14 ÷ 24 is 0.583, and the stamp is rounded to 585 thousandths. So a 14-carat piece is 58.5% pure gold, and it is on that share of the weight that you are paid, not on the gross weight of the piece.
Do you pay on gross weight or on fine weight?
On fine weight. The copper and silver in the alloy have hardly any value, so we do not settle on those. With a 14-carat piece that means 58.5% of the weight on the scales counts towards the price, and you see that step in the calculation.
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