How gold karats work — the complete guide
What does 18K actually mean? Learn how gold purity is measured, why it matters for pricing, and how to identify karats by hallmark.
What is a karat?
In jewellery, a karat is a unit of purity. It expresses how many parts of pure gold are present out of 24 total parts in an alloy. So 24K means 24 parts out of 24 — pure gold — while 18K means 18 parts gold and 6 parts other metals.
The base of 24 was chosen historically because it divides cleanly into halves, thirds, quarters, and sixths — making fractional purities easy to express in pre-decimal trade. Today it's the universal standard, even though gold is also expressed in millesimal fineness (e.g. 750 for 18K, meaning 750 parts per thousand).
The karat purity table
| Karat | Purity | Common name | Typical use |
|---|---|---|---|
| 24K | 99.9% | Fine gold | Bullion, investment |
| 22K | 91.7% | High karat | Indian / Asian jewellery |
| 18K | 75.0% | 18 carat | European fine jewellery |
| 14K | 58.3% | 14 carat | US & middle market |
| 10K | 41.7% | 10 carat | Budget jewellery |
| 9K | 37.5% | 9 carat | UK entry-level |
| 8K | 33.3% | 8 carat | Some EU markets |
How to calculate gold content
To find out how much pure gold is inside any alloyed piece, multiply the total weight by the karat fraction.
Pure gold (g) = Total weight (g) × (Karat ÷ 24)
10 × (18 / 24) = 10 × 0.75 = 7.5 g of pure gold.
The remaining 2.5 g is the alloy (typically copper, silver, or palladium).
Gold hallmarks by country
Most jewellery legally sold in major markets must be hallmarked. The hallmark certifies the purity by an independent assay office or by the maker under regulation.
- UK: Full hallmark with sponsor mark, fineness number (e.g. 750), assay office mark, and date letter.
- EU: CE-equivalent fineness numbers; many countries also use national assay marks.
- Middle East: Arabic numerals for purity (e.g. ٧٥٠ = 750), often plus maker stamp.
- US: Karat stamp (14K, 18K) plus a registered maker mark; assay offices are not used.
Why karat matters for pricing
The pure-gold content is what your refinery or merchant pays for. Two pieces of identical weight but different karats will fetch wildly different prices. A 10 g 18K piece contains 7.5 g of gold; a 10 g 9K piece contains only 3.75 g — half the value at the same weight.
This is why every scrap calculation starts with verifying the karat. See our full guide on pricing scrap gold for the full method.
Try this in the Scrap Gold Calculator
Enter weight and karat, and Goldify gives you the exact buy price using your merchant rate.