Carat to fineness converter
Carats measure gold in twenty-fourths and hallmarks measure it in thousandths, so converting one to the other is a division by 24, except where British law has set its own figure. UK hallmarking law maps 9ct to 375, 14ct to 585, 18ct to 750 and 22ct to 916.6, and those legal standards are what this converter returns for those carats; for any other carat, such as 10, 12, 15 or 21, it gives the plain proportion and says that it is not a UK standard. It also shows the price of a gram at that fineness, from the spot price of gold in pounds read from gold-api.com with its time shown. Use it for old or foreign marks, for a fineness a buyer’s test has found, or simply to know what 18ct means. Gold by Carat is an independent site, not a gold buyer or a dealer.
Example: 18 carat
750 ‰
A standard set by UK hallmarking law
| Gold share | 75.0% |
| k/24 proportion | 750 ‰ |
| Value of 1 g at today’s price | £74.72 |
Figures and rules checked by the site’s publisher, Radif Partners · How the values are worked out · Editorial policy
Carats and fineness in thousandths, side by side
| Carat | Thousandths | Status | Per gram |
|---|---|---|---|
| 8ct | 333.3 | Proportion only | £33.20 |
| 9ct | 375 | UK standard | £37.36 |
| 10ct | 416.7 | Proportion only | £41.51 |
| 12ct | 500 | Proportion only | £49.81 |
| 14ct | 585 | UK standard | £58.28 |
| 15ct | 625 | Proportion only | £62.26 |
| 18ct | 750 | UK standard | £74.72 |
| 20ct | 833.3 | Proportion only | £83.02 |
| 21ct | 875 | Proportion only | £87.17 |
| 22ct | 916.6 | UK standard | £91.31 |
| 24ct | 999 | UK standard | £99.52 |
More detail below: tap a heading to open it.
Where the law departs from the arithmetic
Schedule 1 of UK hallmarking law gives the carat equivalents used in British hallmarking: 9 carat 375, 14 carat 585, 18 carat 750, 22 carat 916.6. Two of them do not match k/24 exactly. Fourteen twenty-fourths is 583.3, so the standard is set higher at 585. Twenty-two twenty-fourths is 916.67, so the standard is set a hair lower at 916.6. The hallmarking law adds 990 and 999 for pure gold. Any other figure is not a British standard and cannot be struck as a fineness mark.
For 24ct, the converter returns 999, the higher of the two pure standards. Bullion bars and Britannias are 999.9, which the gold bar page uses.
Reading foreign and old marks
Numbers you may meet on jewellery bought abroad or inherited include 417 (10 carat) and 875 (21 carat), and older British pieces can be marked 15ct or 12ct, standards no longer in the Act. Each is a claim by the maker; a buyer here will test. Type the carat into the converter to see what it is worth per gram, and see the hallmark guide for what a full British mark looks like.