Understanding Gold Karats and Fineness

GOLD PURITY FOUNDATIONS · 14 MIN READ

Understanding gold karats and fineness

By GoldObserve Research · Published and updated July 28, 2026

Gold karat measures purity in 24 parts: 18K means 18 of 24 parts are gold, or 75%. Millesimal fineness measures purity in 1,000 parts: 750 also means 75% gold. Percentage uses 100 parts, so the same alloy is 75% gold. These systems describe gold content, not item weight, authenticity, color, craftsmanship or retail value. A purity mark is useful evidence, but it is not automatically proof that the object matches it.

INTERACTIVE PURITY WORKBENCH

Convert karat, fineness and gold percentage

Choose a common karat or enter any one purity system. The calculator synchronizes all three formats, converts the selected weight into pure-gold and alloy weight, loads a source-labelled gold reference, estimates metal value, and translates common numerical or surface-layer stamp clues.

CORE CONVERSIONS

One purity expressed in three different scales

COMMON GOLD GRADES

Karat, theoretical fineness and familiar marks

“Corresponding” does not always mean mathematically identical. Fourteen parts out of 24 equals about 583.333 parts per thousand, while a 585 mark states 585. A governing hallmark standard, assay or product specification should take priority over a generic conversion table.

KARAT VS. CARAT

Gold purity and gemstone weight are different measurements

NIST notes that the gemstone carat is standardized at 0.2 gram. It should never be inserted into a gold karat calculation. When a document uses “carat” for gold fineness, the 24-part context must be clear.

READING MARKS

A number can describe purity, but the complete mark tells more

A bare number such as 750 is commonly read as 750 parts gold per thousand, or 18K. In a regulated hallmark system, a complete hallmark may also identify the sponsor or maker and the assay office. The UK hallmarking guidance, for example, explains that a complete hallmark contains multiple compulsory components rather than one isolated purity number.

SOLID ALLOY VS. SURFACE LAYER

Do not apply a karat stamp to the full weight when a layer marker is present

Marks such as GP, GEP, HGE, GF or RGP commonly signal plating, electroplating, heavy electroplate, gold-filled or rolled-gold construction. Terminology and legal definitions vary by jurisdiction, so the purchase document and local rules matter. The crucial valuation point is universal: a surface layer cannot be valued by multiplying the entire object weight by the plating karat.

The FTC distinguishes karat-gold items from gold-plated, gold-filled, overlay and rolled-gold products. Treat the full wording as a construction description, not as a single purity number. The gold jewelry construction guide separates solid, hollow, plated, filled and vermeil evidence before any value calculation.

COLOR AND DURABILITY

Yellow, white and rose color do not determine karat

Gold alloys combine gold with other metals to change color, hardness, workability and wear behavior. Two 18K objects can both contain 75% gold while using different alloy recipes and appearing different in color. Likewise, a yellow object is not necessarily high-karat gold, and a white object is not necessarily white gold. Color, magnet response and surface appearance are screening clues at most, not purity measurements.

FROM PURITY TO VALUE

Worked example: 12 grams marked 585

Stones, non-gold parts, solder, hollow construction, mixed-metal sections, assay variance and dealer terms can change a real transaction. The example demonstrates the purity arithmetic, not authenticity or a guaranteed payout.

VERIFICATION

A stamp is a starting point, not the end of due diligence

• Read the entire mark, including letters before and after the purity number.
• Separate maker, sponsor, assay-office and date marks from fineness marks.
• Check the item description, receipt and provenance against the physical mark.
• Do not infer purity from color, density, magnet response or one home test alone.
• Use an appropriate professional test or assay when the transaction value justifies it.
• Record any uncertainty rather than converting an unresolved mark into a precise value.

Hallmarking law and permitted descriptions differ across countries. GoldObserve translates numerical formats; it does not certify an item or determine which local legal standard applies.

PRIMARY SOURCES & METHOD

Standards and consumer guidance used

• FTC guidance on buying gold jewelry for the 24-part karat explanation and surface-layer terminology.
• British Hallmarking Council guidance for millesimal fineness and the components of a complete UK hallmark.
• NIST precious-metals conversion information for the distinction between gold purity context and the 0.2-gram gemstone carat.
• GoldObserve methodology for purity, unit, price and rounding order.

Calculator conversions use karat divided by 24, fineness divided by 1,000, or percentage divided by 100. They translate the entered claim and do not authenticate the material.

RELATED TOOLS & GUIDES

Continue from a purity mark to a defensible valuation