A ring listed as “18K, 1 ct” uses two words that sound the same and measure completely different things. The carat after the diamond is a unit of weight. The karat on the band is a statement of purity — how much of the metal is gold. Neither tells you anything about the other, and mixing them up is how people end up thinking a bigger number on the band means a bigger stone, or that a one-carat stone is somehow one twenty-fourth of something.
Carat: a unit of weight, fixed at exactly 0.2 grams
The metric carat is defined as exactly 200 milligrams — one fifth of a gram — and has been since it was adopted internationally in 1907. Before that, the carat varied from one trading city to the next, which made the same stone a slightly different weight depending on where it was sold. Fixing it to the gram ended that.
A carat divides into 100 points, so a 0.25 ct stone is a “twenty-five pointer” and a 0.07 ct accent stone is seven points. When a piece holds several stones, the tag usually gives the combined figure as ctw or tcw, total carat weight: “1 ctw” on a cluster ring might be twenty stones of five points each, which is a very different thing from a single one-carat stone. Pearls are often weighed in a different unit again, the pearl grain, which is a quarter of a carat.
Because carat is weight, it is not size. Stones of different materials have different densities, so a one-carat ruby is physically smaller than a one-carat diamond, and a one-carat emerald larger. Even within diamond, cut matters: a deep stone hides weight below the girdle where it does not show. As a rough guide, the trade’s standard estimation factor puts a well-cut one-carat round brilliant at about 6.4 mm across. The diamond size calculator runs that arithmetic both ways, and the carat to gram converter does the unit conversion, which is exact.
Where the carat came from — and a myth worth dropping
The word comes, through Arabic and Italian, from the Greek keration, the seed of the carob tree. The story usually told is that carob seeds are so uniform in weight that traders used them as counterweights. A 2006 study in Biology Letters by Turnbull and colleagues tested that by weighing carob seeds, and found they vary about as much as the seeds of any other plant. Their suggestion was that traders picked out seeds that looked alike, which people can do by eye to within a few percent — so any uniformity was human selection, not botany. The metric carat made the question moot in 1907.
Karat: purity, counted in twenty-fourths
Karat measures the proportion of gold in an alloy, on a scale where 24 is pure. One karat is one twenty-fourth part by weight. So 18 karat is 18/24 gold, exactly 75%; 14 karat is 14/24, or 58.33%; 10 karat is 10/24, or 41.67%; and 24 karat is pure gold. The rest of the metal is other metals — usually silver, copper, zinc, nickel or palladium — chosen for colour and hardness.
Many countries stamp purity as parts per thousand instead: 750 for 18 karat, 585 for 14 karat, 375 for 9 karat. These are close to the fractions but not always identical — 585 is slightly more than 14/24, which works out to 583.3. In the US, the law allows a stamp a small shortfall below its mark, and the gold stamp tolerance calculator shows exactly how much. The gold purity converter moves between karats, parts per thousand and percentages.
The spelling trap: carat means both in British English
In American English the split is tidy: carat for stones, karat for gold, abbreviated ct and K or kt. In British English, “carat” is used for both, so “9 carat gold” and “a 1 carat diamond” are both normal UK usage. The abbreviation usually gives it away: a “9ct” ring in a UK shop means 9 carat gold, not a nine-carat stone. Context settles it — a number on a metal describes purity, and a number on a stone describes weight.
Reading both on one tag
Take a tag that says “14K white gold, 0.50 ctw”. The first half is about the metal: the band is 14 karat, so 14/24 of its weight is gold. The second half is about the stones: their weights add up to half a carat, or 0.1 g, across however many there are. The tag says nothing about how much the band weighs, and the band’s weight is what you would need, with its karat, to work out how much gold it contains. Melt value follows from that — and only from that, because it ignores the stones entirely, and neither melt value nor carat weight is the same as what the piece would sell for.
One last distinction: a higher karat is not a better piece, and a bigger carat weight is not a better stone. Higher karat gold is softer and scratches more easily, which is why much everyday jewellery is 14 or 18 karat. And a stone’s weight is only one of the factors that describe it — the others are covered in the four Cs explained.