A carat is not a karat
Two words, one pronunciation, two entirely different quantities — and outside North America they are not even spelt differently.
The two meanings, side by side | Carat (ct) | Karat (K) |
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| Measures | Mass of a gemstone | Purity of a gold alloy |
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| Value | Exactly 0.2 g | Twenty-fourths of pure gold |
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| Range | Any positive number | 1 to 24 |
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| Spelt “karat” | Never | US and Canada only |
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So “a two carat diamond in an eighteen carat setting” is a perfectly ordinary British sentence using the word twice for two unrelated things. The stone weighs 0.4 g; the setting is three-quarters gold.
Both words come from the carob seed, once used as a small standard of weight, and the purity sense arrived by a separate route — a coin of 24 carats of gold in a mark of alloy. They have been distinct for centuries and the spelling only split in one market.
This converter keeps them apart deliberately: the gemstone carat appears in the weight list, purity has its own control, and nothing lets one be entered where the other belongs. A tool with a single “carat” field would produce a confident answer to a question nobody asked.
916 is 22 karat, written differently
The number stamped on Indian jewellery is not a separate grading system. It is the karat expressed as parts per thousand: 22 divided by 24 is 0.9167, so 22 karat is stamped 916.
Every ordinary grade, in both notations| Karat | Hallmark | Gold | Where it is standard |
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| 24K | 999 | 99.9% | Bullion and Asian investment pieces |
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| 22K | 916 | 91.7% | Indian jewellery |
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| 21K | 875 | 87.5% | The Gulf |
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| 18K | 750 | 75.0% | European and American fine jewellery |
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| 14K | 585 | 58.3% | US mass market |
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| 10K | 417 | 41.7% | US legal minimum |
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| 9K | 375 | 37.5% | UK legal minimum |
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Once the pattern is visible every hallmark reads directly. 750 is three quarters, so 18 karat. 375 is three eighths, so 9. The stamps that look slightly off — 585 for 14 karat rather than 583, and 417 for 10 rather than 416.7 — are rounded to three figures in the direction the standard permits.
Two rows are worth pausing on. 24 karat is hallmarked 999 rather than 1000 because absolute purity is not achievable; refiners quote 999.9 or 999.99 for investment bars. And the last two rows are legal thresholds rather than preferences: an article below 375 cannot be sold as gold in the UK, and below 10 karat it cannot be called gold in the United States — so the same 9 karat chain is legally gold in London and not in New York.
The alloy is not filler in any pejorative sense. Pure gold is too soft for most jewellery; the copper and silver in a 22 or 18 karat alloy give it wearability, and the choice of alloy is what makes white, rose and yellow gold of the same karat different colours.
A troy ounce is not an ounce
Precious metals are weighed in troy, and everything else is weighed in avoirdupois. The two ounces share a name and differ by nearly a tenth.
The two ounces, and the units built on them| Unit | Grams | Defined as |
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| Troy ounce | 31.1035 | 480 grains |
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| Ordinary ounce | 28.3495 | 1/16 pound |
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| Tola | 11.6638 | 180 grains |
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| Pennyweight | 1.5552 | 24 grains |
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| Gemstone carat | 0.2000 | Exactly 200 mg |
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The troy ounce is 9.71% heavier. Converting a gold quotation with the ordinary ounce loses a tenth of the value, in the direction of under-stating it — which is the more dangerous direction if you are selling.
A satisfying consequence falls out of the grain: a tola is 180 grains and a troy ounce is 480, so a tola is exactly three eighths of a troy ounce. A price of 2400 per ounce is exactly 900 per tola, with no rounding anywhere. That relationship is why the tola survived alongside the troy ounce in markets that traded with both.
The traditional subdivisions keep the same exactness. A masha is a twelfth of a tola and a ratti an eighth of a masha, so 96 ratti make a tola. They are rarely used for gold now but still turn up in gemstone dealing in South Asia, where a ratti is close enough to the metric carat — 0.1215 g against 0.2 — to be worth being careful about.
Gross weight is not gold weight
A scale reads the whole item. Only part of it is gold, and how much depends entirely on the karat.
What must be on the scale to hold 9.167 g of gold| Purity | Gross weight | Against 22 karat |
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| 24 karat | 9.17 g | 0.92× |
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| 22 karat | 10.00 g | 1.00× |
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| 18 karat | 12.22 g | 1.22× |
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| 14 karat | 15.71 g | 1.57× |
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| 9 karat | 24.44 g | 2.44× |
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Every row holds the same gold and would fetch the same metal price. The 9 karat piece is nearly two and a half times as heavy as the 22 karat one — so comparing two pieces of jewellery by what the scale says, without the karat, tells you nothing at all about their value.
It also explains a common surprise when selling. A heavy-feeling 14 karat chain can be worth less than a lighter 22 karat one, and the difference is not the buyer being unfair; it is 58% against 92%. The useful habit is to convert both to gold content before comparing anything.
One caution the arithmetic cannot cover: a hallmark is a statement about the alloy the piece was made from, not a guarantee about the object in your hand. Re-worked, repaired, unhallmarked or imported items may not match their stamp, and only an assay settles it.
Reading a quotation
Gold is quoted internationally per troy ounce of pure metal, and retail rates in each market are derived from it in a local unit. Knowing which unit and which purity a number refers to resolves most of the confusion.
India quotes retail per 10 g, and quotes it separately for 22 karat and 24 karat — so two numbers from the same shop on the same day are both correct and describe different metal. The 22 karat rate is roughly 91.7% of the 24 karat one before any charges.
The Gulf and Pakistan use the tola, and because a tola is exactly three eighths of a troy ounce the conversion is clean arithmetic rather than a decimal.
International markets quote the troy ounce for 24 karat, which is the number that appears in news reports and is never what a jeweller charges.
What separates a metal value from a retail price is everything a jeweller adds: making charges — often a percentage of the metal value or a rate per gram — wastage, the cost of any stones, and tax. In the other direction, a buyer purchasing scrap offers below spot and may deduct refining. The figure this page produces is the metal content, which is the right starting point for both conversations and is not the price in either.
Sources and methodology
The weights on this page are exact rather than measured. The troy ounce has been exactly 480 grains and the pound exactly 453.59237 g since 1959, and the metric carat has been exactly 200 mg since 1907 — so the troy premium, the tola and the gemstone carat all follow from definitions with no experimental uncertainty in them. The hallmark grades are those of the Indian and UK standards, which differ in what may legally be called gold at all.