How to Convert Between Karat, Fineness, and Purity Percent

You’re looking at a piece of jewelry stamped “18K” and trying to figure out what that actually means in terms of purity percentage, or comparing it against a piece marked “750” — and realizing gold, silver, and platinum purity gets expressed in at least three different, genuinely incompatible-looking notations depending on where and how the piece was made: karat (a fraction out of 24), millesimal fineness (parts per thousand), and straightforward percentage purity. They’re all describing the same underlying concept, just in different units, which makes direct comparison confusing without converting them to a common notation first.

Sterling silver’s familiar “925” stamp is actually the same kind of millesimal fineness notation as gold’s “750” — both indicate parts pure metal per thousand, once you know how to read it.

What converting metal purity actually involves

Karat notation expresses purity as a fraction of 24 (24 karat being pure gold), so 18K gold is 18/24, or 75% pure. Millesimal fineness expresses the same purity as parts per thousand — 750 fineness meaning 750 parts pure metal per 1000, which is again 75%. Percentage purity is the most directly readable of the three, simply stating the percentage of pure metal by weight. Converting between them is straightforward once you know each notation’s underlying basis, but the mental math to do it correctly on the spot isn’t always obvious, especially for less common karat values.

Why people get stuck here

• Not knowing karat is a fraction of 24, not some independent scale. Without knowing the base is 24, converting a karat value to a percentage or fineness figure isn’t obvious — 18K specifically means 18 parts out of 24, not simply “18%.”
• Confusing millesimal fineness with a completely different measurement system. A stamp like “925” or “750” looks like an arbitrary code without knowing it’s simply parts-per-thousand purity, directly convertible to a percentage by moving a decimal point.
• Comparing pieces stamped in different notations without converting first. A piece marked in karat and another marked in fineness aren’t directly comparable at a glance without converting both to the same notation.
• Not accounting for alloy composition when purity matters for value or use. Purity notation tells you the proportion of pure precious metal, but doesn’t say what the remaining alloy is made of, which can matter for durability, color, and hypoallergenic properties.

What a good metal purity converter looks like

Converts cleanly between all three common notations

Handling karat, millesimal fineness, and percentage together, in any direction, removes the need to work out the underlying math manually each time.

Includes standard hallmark presets

Providing common standard purity marks (like 10K, 14K, 18K, 24K for gold, or 925 sterling for silver) as quick presets makes checking a familiar stamped value fast and simple.

Shows alloy composition breakdown

Displaying what portion of the total is the base precious metal versus alloy gives useful additional context beyond the raw purity number alone.

Common mistakes to avoid

• Assuming karat notation is a percentage directly, rather than a fraction specifically out of 24.
• Comparing two purity-stamped pieces in different notations without converting them to the same basis first.
• Confusing a fineness stamp like “925” with an arbitrary code rather than recognizing it as straightforward parts-per-thousand purity.
• Assuming higher purity always means better for every use — softer, higher-purity metal can actually be less durable for daily-wear jewelry compared to a lower-purity alloy.
• Not checking which specific alloy metals make up the non-pure portion when that composition matters, such as for allergy sensitivity.

How to do it with Metal Purity Converter

Online Tool Store’s Metal Purity Converter converts your value entirely in your browser.

1. Open the Metal Purity Converter tool.
2. Enter a karat, fineness, or percentage value, or select a standard hallmark preset.
3. Get the converted value in the other two notations instantly.
4. Review the alloy composition breakdown for additional context.

Frequently asked questions

What does “18K” actually mean in percentage terms?

18 karat means 18 parts pure gold out of 24 total parts, which works out to 75% pure gold by weight — the remaining 25% is alloy metal, commonly copper, silver, or other metals depending on the desired color and properties.

Is “925” silver the same idea as karat gold notation?

It’s the same underlying concept (a purity fraction) expressed in a different notation — 925 is millesimal fineness, meaning 925 parts pure silver per 1000, or 92.5% pure, which is the standard definition of sterling silver.

Does higher purity always mean a better piece of jewelry?

Not necessarily for durability — pure precious metals are generally softer, so higher-purity gold or silver can be more prone to scratching and denting in daily-wear jewelry compared to a lower-purity alloy, which is part of why 14K or 18K gold is more common for rings than 24K.

Final thought

Karat, fineness, and percentage are all describing the same underlying purity, just in different notations — convert to a common one before comparing two pieces, and the numbers actually become meaningful.

Try the free Metal Purity Converter tool