Understanding Gold Purity: 24K, 22K, 18K Explained

The Short Answer

Karat measures how much of an alloy is gold, counted in twenty-fourths: 24K is at least 99.9 percent pure, 22K is 91.67 percent, 18K is 75 percent and 9K is 37.5 percent, with the balance made up of harder metals. Fineness says the same thing in parts per thousand — 999, 916, 750, 375 — and is what assay offices stamp. Investment bullion is struck at 999 or 999.9 because purity is the whole point; jewellery is usually 18K or lower because the alloy is what lets it survive daily wear.

For bullion, price follows fine gold content rather than the karat stamp: a 22K one-ounce coin and a 24K one-ounce coin each hold one troy ounce of gold and trade within a few percent of each other. For jewellery, karat sets the metal value but not the price, which is mostly labour, brand and tax. The chart below converts between the systems; the sections after it cover what each level is used for, the legal thresholds that matter, and how purity is actually checked.

Gold Purity Chart: Karat to Fineness

The arithmetic is simple: divide the karat by 24. Eighteen divided by twenty-four is 0.75, so 18K is 750 fine. The United Kingdom recognises six legal standards for gold — 375, 585, 750, 916.6, 990 and 999 — and a British hallmark guarantees the whole article is at least that fine. Other countries use overlapping but not identical lists, so a stamp is only as good as the system behind it; our guide to international hallmarks covers the main ones.

24 Karat Gold: The Investment Standard

At 999 or 999.9 fine, 24 karat gold is as close to the pure element as refining makes practical. It is deep yellow, soft enough to mark with a fingernail’s pressure over time, and impractical for jewellery that will be worn — which is exactly why it is the standard for bullion, where the aim is the maximum fine gold per gram of metal handled.

The mainstream 24 karat products are the Royal Mint’s gold Britannia (999.9 fine, 31.21 g, £100 face value), the Royal Canadian Mint’s Gold Maple Leaf (.9999, struck since 1979) and bars from LBMA-accredited refiners such as PAMP Suisse and Valcambi. For the investor the valuation is the simplest in the market: fine weight multiplied by the spot price, plus the dealer’s premium. The difference between three nines and four nines is real to a refiner and almost invisible to a private buyer — both trade against the same spot price — a point our piece on what carat 999.9 gold is takes further.

22 Karat Gold: Durability Meets Tradition

The American Gold Eagle, South African Krugerrand and British Sovereign are struck at 22 karat: 916.7 parts per thousand, with the balance copper and silver. The alloy is markedly harder than pure gold, so these coins tolerate handling, stacking and loose storage that would mar a 24K surface. The Krugerrand’s warm reddish tone comes from its copper content.

The point that confuses new buyers is that a one-ounce 22K coin still contains a full troy ounce of fine gold. The coin is heavier than an ounce overall because the alloy is additional metal, not a substitute. Pricing is on gold content, so an Eagle and a Maple Leaf hold the same gold despite different gross weights. For tax the distinction is irrelevant too: Article 344 of the EU VAT Directive sets the coin threshold at 900 thousandths, so 916.7 qualifies as investment gold just as 999.9 does. Outside bullion, 22K is the traditional standard for wedding and festival jewellery across India and the Gulf, where the metal value of the piece is part of its purpose.

18 Karat and Below: The Jewellery Standards

Eighteen karat — 750 fine, 75 percent gold — is the international standard for fine jewellery and for gold watch cases. It balances colour against the hardness needed for prongs, clasps and hinges. The remaining quarter of the alloy sets the colour: copper for rose gold, palladium or nickel for white gold, silver and zinc to brighten yellow. Our comparison of rose, yellow and white gold covers the trade-offs.

Below 18K sit 14K (585, the common standard in the United States and much of Northern Europe), 10K (417) and 9K (375, the British and Australian high-street norm). They are harder and cheaper, and they contain proportionally less gold, so the metal value of a 9K chain is a little over a third of the same weight in pure gold. None of them meets the 995 bar or 900 coin tests, so none is investment gold for VAT purposes, and the gap between what jewellery costs and what its metal is worth is wider the lower the karat — the subject of why jewellery loses more than bullion. For the jewellery buyer’s view of these grades, our 9k to 24k purity guide goes into wear and colour in detail.

How Purity Sets Metal Value: A Worked Example

Take 10 grams of metal at each standard and an illustrative spot price of 100 currency units per gram of fine gold (the figure is for arithmetic only — use the live price when you calculate).

• 24K (999): 9.99 g fine gold, metal value 999
• 22K (916.7): 9.17 g fine gold, metal value 917
• 18K (750): 7.50 g fine gold, metal value 750
• 14K (585): 5.85 g fine gold, metal value 585
• 9K (375): 3.75 g fine gold, metal value 375

That is what a refiner or a scrap buyer starts from, before deducting their margin. It is not what a jeweller charges, because a finished piece carries design, labour, brand and sales tax on top, and it is not what a bullion dealer charges either, because a coin or bar carries a premium over spot. When you compare any two gold products, convert both to a price per gram of fine gold first — our bullion buying guide shows the method for coins and bars.

The Thresholds That Matter: 995, 900 and Good Delivery

Three numbers recur in gold documentation and are worth knowing by heart. 995 is the minimum fineness for a London Good Delivery bar — the roughly 400 oz wholesale bar, containing between 350 and 430 fine troy ounces, that settles trades in the London market — and it is also the purity floor for a bar or wafer to count as investment gold under Article 344 of the EU VAT Directive. 900 is the floor for a coin under the same Article, provided it was minted after 1800, is or was legal tender in its country of origin, and normally sells for no more than 80 percent above its gold value. 999.9 is a refining standard, not a legal one: four-nines bars and coins are the norm because the major refiners can produce them, not because the law demands them.

Products from LBMA-accredited refiners at 995 or better are accepted across the wholesale market on the strength of the refiner’s mark. Lower-purity items, and items from unknown makers, may need re-assaying before a buyer will pay full value, which costs time and money at exactly the moment you want to sell.

How Gold Purity Is Checked

A stamp is a claim, and claims are tested in three tiers. At home, a strong magnet rules out ferrous fakes, a 0.01 g scale and a water-displacement density test show whether the weight is consistent with the stated karat, and an acid kit distinguishes 10K, 14K, 18K and 22K by colour reaction — none of which produces a percentage. At a jeweller or bullion dealer, an XRF analyser reads the surface alloy in seconds without damage and gives actual percentages. At an assay office, fire assay or ICP analysis destroys a tiny sample and returns the definitive figure, which is what stands behind a hallmark.

For anything of real value, stop at tier two and pay for the reading. Our tutorial on how to test gold at home ranks the five home methods by what each one proves, and our hallmarks explainer shows how to read the marks that an assay office leaves behind.

Sources

• legislation.gov.uk/uksi/1998/2978/regulation/2/made/data.xht
• legislation.gov.uk/eudr/2006/112/article/344
• lbma.org.uk/publications/the-otc-guide/london-good-delivery-gold-and-silver
• royalmint.com/collect/archive/britannia-2018-one-ounce-gold-bullion-coin
• mint.ca/en/shop/bullion-products/2019-50-1-oz-9999-pure-gold-40th-anniversary-of-the-gml

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