Gold purity explained: karat, fineness, and what each alloy does in the file
What 9ct, 14k, 18k, 22k and 916 actually mean, how the numbers convert, and why the alloy changes the model as well as the price.
Gold purity is quoted in at least three different systems depending on where you are standing, and clients rarely use the same one you do. The conversions are simple once you have seen them, and the choice of alloy affects more than cost — it changes the weight, the colour, and what the metal will tolerate at the bench.
Karat and fineness are the same number twice
Karat expresses gold content in twenty-fourths. Fineness expresses it in parts per thousand. Pure gold is 24 karat, or 1000 fine.
To convert karat to fineness, divide by 24 and multiply by 1000. To go the other way, divide fineness by 1000 and multiply by 24.
The table worth keeping
| Karat | Fineness | Gold % | Where it is common |
|---|---|---|---|
| 9ct | 375 | 37.5% | UK; rare in the US |
| 10k | 417 | 41.7% | US legal minimum for "gold" |
| 14k | 585 | 58.5% | The American workhorse |
| 18ct / 18k | 750 | 75.0% | International fine jewellery standard |
| 22ct | 916 | 91.6% | South Asia and the Middle East |
| 24ct | 999 | 99.9% | Bullion rather than jewellery |
The Indian 916 hallmark and 22ct are the same thing said two ways, which is worth knowing when a client quotes one and your caster quotes the other.
Purity drives weight, and weight drives cost
Higher karat means more gold and therefore denser metal. The same model, unchanged, weighs noticeably more in 22ct than in 9ct.
Approximate densities in grams per cubic centimetre: 9ct around 11.3, 14k around 13.1, 18ct yellow around 15.5, 22ct around 17.8. A shape that comes out at 5g in 9ct is closer to 7.9g in 22ct — before you have touched the design.
So when a client changes their mind about karat, the metal cost moves twice: the gold content per gram rises, and the number of grams rises too.
What the alloy does at the bench
Purity is not only a price decision. It changes how the metal behaves.
- Higher karat is softer. 22ct scratches and bends more readily, and prongs wear faster.
- Lower karat is harder and more durable, which is why 9ct and 14k suit heavy everyday wear.
- 22ct is more forgiving to set into, because the metal moves easily under the setter's tool.
- 9ct is more prone to tarnish over time, because there is more base metal in the mix.
For a piece with fine prongs in 22ct, expect to model them heavier than you would in 14k. The metal will not hold a thin prong the same way.
Colour comes from what is not gold
Yellow, white and rose are all the same gold content at a given karat — the difference is the alloying metals.
- Yellow: copper and silver in balance.
- Rose: more copper, less silver. Harder than yellow at the same karat.
- White: alloyed with palladium or nickel, and usually rhodium plated. Density varies widely, roughly 14.7 to 16.0 at 18ct.
White gold is the one to confirm rather than assume. A nickel white and a palladium white at the same karat can differ enough in density to matter on a heavy piece, and nickel raises an allergy question in some markets.
Hallmarking
Requirements differ by country. The UK requires assay above weight thresholds and stamps a fineness mark; India's BIS hallmark carries the purity as a three-digit number such as 750 or 916; the US relies on karat stamps without a compulsory assay office. If a piece is crossing borders, the mark it needs is worth checking before it is made.
Confirm the alloy before the model is finished
Karat is usually treated as a late decision, and it should not be. It changes the weight of the piece, the cost of the metal, and in some cases the geometry the design needs to survive wear.
We report an estimated weight in your chosen alloy with every commission, so the figure is in front of you before anything is cast. Quotations are free — send the design and the karat, and we will tell you what it will weigh.
