Gold Purity Explained: 9ct vs 18ct vs 24ct
Understanding gold purity is essential whether you're buying jewelry, selling scrap gold, or investing in bullion. This guide explains what the numbers mean, how alloys create different colours, and which purity is right for your needs.
The Quick Answer
24 carat = 99.9% pure gold. Divide any carat by 24 to get the gold percentage. So 18ct = 18/24 = 75% gold, and 9ct = 9/24 = 37.5% gold. The three-digit hallmark number (375, 750, 916, 999) shows parts per thousand of pure gold.
Why is 24 Carat "Pure" Gold?
The 24-part system has its roots in medieval Germany. A gold coin called the "mark" weighed 24 carats - an ancient unit of weight derived from carob seeds, which were remarkably consistent in mass and used as balance weights.
If all 24 parts of the mark were pure gold, it was "24 carat". If 18 parts were gold and 6 parts were other metals, it was "18 carat". This system stuck, even though we no longer use the carat as a unit of weight for anything except gemstones.
Carat vs Karat
In the UK, we use "carat" (ct) for gold purity. Americans use "karat" (K or kt). They mean the same thing: 18ct = 18K = 75% gold. Don't confuse this with gemstone carats, which measure weight (1 carat = 0.2 grams).
Understanding Fineness (The Numbers)
Modern hallmarks use millesimal fineness - parts per thousand of pure gold. This is more precise than the carat system and is the international standard.
| Carat | Fineness | % Gold | Calculation |
|---|---|---|---|
| 24ct | 999 | 99.9% | 24 ÷ 24 = 1.00 |
| 22ct | 916 | 91.6% | 22 ÷ 24 = 0.916 |
| 18ct | 750 | 75% | 18 ÷ 24 = 0.75 |
| 14ct | 585 | 58.5% | 14 ÷ 24 = 0.583 |
| 9ct | 375 | 37.5% | 9 ÷ 24 = 0.375 |
Quick Tip: Reading Hallmarks
If you see 375 stamped on jewelry, that's 9ct gold. 750 is 18ct.916 is 22ct (common on Sovereigns). For a full guide to reading UK hallmarks, see our Hallmarks Guide.
Comparing Gold Purities
Each purity level has distinct characteristics. Here's how they compare:
Bright, rich yellow
Very soft - scratches and bends easily
- • Investment bars
- • Bullion coins
- • Some Asian jewelry
- ✓ Highest gold content
- ✓ Best for investment
- ✓ Hypoallergenic
- ✓ Never tarnishes
- ✗ Too soft for most jewelry
- ✗ Scratches easily
- ✗ Higher price point
Deep, warm yellow
Soft - not ideal for everyday wear
- • Gold Sovereigns
- • Krugerrands
- • Traditional Asian jewelry
- • Wedding bands (India)
- ✓ High gold content
- ✓ Beautiful colour
- ✓ Legal tender coins are CGT-free
- ✗ Still quite soft
- ✗ Limited UK jewelry selection
- ✗ Premium pricing
Rich yellow (or white/rose depending on alloy)
Good balance - suitable for fine jewelry
- • Engagement rings
- • Wedding bands
- • Fine jewelry
- • Luxury watches
- ✓ Beautiful colour
- ✓ Good durability
- ✓ Prestigious
- ✓ Wide selection
- ✗ More expensive than 9ct
- ✗ Can still scratch
- ✗ May cause allergies (nickel in white gold)
Lighter yellow than 18ct
Very durable - excellent for daily wear
- • American jewelry standard
- • Engagement rings
- • Fashion jewelry
- ✓ Good balance of price and purity
- ✓ Durable
- ✓ Less likely to trigger allergies than 9ct
- ✗ Less common in UK
- ✗ Paler colour than 18ct
Paler, lighter yellow
Very hard and durable - best for everyday wear
- • Everyday jewelry
- • Chains
- • Earrings
- • Budget-friendly pieces
- ✓ Most affordable
- ✓ Very durable
- ✓ Scratch resistant
- ✓ Wide availability
- ✗ Lower gold content
- ✗ Can tarnish over time
- ✗ May cause allergies
- ✗ Paler colour
The Chemistry of Gold Alloys
Pure gold is too soft for most jewelry. By adding other metals, jewellers create alloys that are harder, more durable, and come in different colours. The specific metals used determine the final appearance.
| Alloy Type | Gold % | Silver % | Copper % | Other Metals | Result |
|---|---|---|---|---|---|
| Yellow Gold (18ct) | 75% | 12.5% | 12.5% | - | Classic warm yellow |
| Rose Gold (18ct) | 75% | 2.5% | 22.5% | - | Pink/copper tone |
| White Gold (18ct) | 75% | 0% | 0% | Palladium 25% or Nickel/Zinc mix | Silvery white (often rhodium plated) |
| Green Gold (18ct) | 75% | 25% | 0% | - | Subtle greenish tint |
| 9ct Yellow | 37.5% | 7.5% | 42.5% | Zinc 12.5% | Paler yellow, can appear slightly greenish |
Copper gives rose gold its distinctive pink hue. The more copper in the alloy, the redder the gold appears. A typical 18ct rose gold contains about 75% gold and 22.5% copper. "Red gold" has even more copper for a deeper colour.
White gold isn't naturally bright white - it has a slightly yellowish or greyish tint. Most white gold jewelry is rhodium plated to achieve that bright, silvery finish. This plating wears off over time and may need reapplication.
Allergy Warning: Nickel in Gold
Some white gold alloys contain nickel, which causes allergic reactions in 10-15% of people. If you have sensitive skin, look for nickel-free white gold (made with palladium) or choose yellow/rose gold instead. Higher purity gold (18ct+) generally contains less allergenic metals than 9ct.
How Purity Affects Your Gold's Value
When selling gold, purity is the primary factor in determining value. The base metals mixed with gold (silver, copper, zinc) have negligible value compared to gold itself.
Example: A 10g 9ct gold chain when gold is £60/gram:
Pure gold value = 10g × £60 × 0.375 = £225
Note: Dealers typically pay 70-90% of this calculated value to cover refining costs and profit.
Value Comparison at Same Weight
For two identical 10g chains - one 9ct and one 18ct - the 18ct chain is worth exactly twice as much for its gold content (75% ÷ 37.5% = 2).
This is why dealers care most about weight and purity, not the item's original purchase price or sentimental value. Use our gold calculator to check current values for any weight and purity.
Which Gold Purity Should You Choose?
9ct or 14ct is ideal. These are hard enough to withstand daily wear, resist scratches, and are more affordable. Perfect for:
- • Wedding bands worn daily
- • Chains and bracelets
- • Earrings for regular wear
- • Budget-conscious purchases
18ct offers a beautiful balance of purity and durability. The richer colour and prestige make it ideal for:
- • Engagement rings
- • Special occasion pieces
- • Heirloom jewelry
- • Investment pieces you'll keep
22ct or 24ct maximises your gold content per pound spent. Choose:
- • Gold bars (999.9) - lowest premiums
- • Britannias (999.9) - CGT-free
- • Sovereigns (916) - CGT-free, historical value
See our investment guide for more on gold investment options.
18ct or higher yellow gold is safest. Higher gold content means fewer allergenic metals. Avoid:
- ✗ White gold with nickel
- ✗ 9ct gold (high base metal content)
- ✓ 18ct+ yellow gold (safest)
- ✓ Palladium white gold (nickel-free)
UK Legal Standards
In the United Kingdom, the minimum purity for an item to be legally sold as "gold" is 9 carats (375 fineness). This is governed by the Hallmarking Act 1973.
UK Recognised Purities
- • 375 (9 carat)
- • 585 (14 carat)
- • 750 (18 carat)
- • 916 (22 carat)
- • 990 (23.76 carat)
- • 999 (24 carat)
Hallmarking Requirements
- • Items over 1g must be hallmarked
- • Items under 1g are exempt
- • Four UK assay offices can hallmark
- • Foreign marks may be accepted
Frequently Asked Questions
What does 375 mean on gold?
375 means 37.5% pure gold, which is 9 carat gold. The number represents parts per thousand - so 375 parts out of 1000 are pure gold, with the remaining 625 parts being other metals like copper, silver, or zinc.
Is 9ct gold real gold?
Yes, 9ct gold is absolutely real gold. It contains 37.5% pure gold alloyed with other metals. In the UK, 9ct is the minimum purity that can legally be sold as "gold". It's the most popular choice for everyday jewelry due to its durability and affordability.
What is the difference between 9ct and 18ct gold?
9ct gold contains 37.5% pure gold while 18ct contains 75% pure gold - exactly double. 18ct has a richer yellow colour and is more valuable, but it's softer and more prone to scratches. 9ct is harder, more durable, and costs roughly half as much per gram.
What makes rose gold pink?
Rose gold gets its pink colour from a higher proportion of copper in the alloy. A typical 18ct rose gold might be 75% gold, 22.5% copper, and 2.5% silver. The more copper, the redder the gold appears.
Which gold purity is best for investment?
For investment, 24ct (999) or 22ct (916) gold is preferred as the value is almost entirely in the metal content. Investment bars are typically 999.9 fine, while coins like Sovereigns and Britannias are 22ct and 24ct respectively. Both UK coins are CGT-exempt, making them particularly attractive for investors.
Can 9ct gold tarnish?
Yes, 9ct gold can tarnish over time because it contains 62.5% other metals (mainly copper and silver) that can oxidise. Higher purity gold is more resistant to tarnishing. Regular cleaning and proper storage help prevent tarnishing on lower carat pieces.
Calculate Your Gold's Value
Now you understand purity, find out what your gold is worth today
Related Guides
Learn to read hallmarks, identify assay office symbols, and verify authenticity.
Read Guide →Step-by-step guide to selling your gold jewelry and getting the best price.
Read Guide →Compare physical gold, ETFs, mining stocks, and other ways to invest.
Read Guide →Sources and References
UK hallmarking standards: The Hallmarking Act 1973. Source:legislation.gov.uk
Assay office information:London Assay Office,Birmingham Assay Office
Last updated: February 2026
Founder & Market Researcher
Taro has been actively investing in precious metals and financial markets for over 15 years. Frustrated by the lack of transparent, accurate gold pricing information in the UK, he built London Gold Exchange as a data-driven resource for fellow investors. The site combines real-time market data, verified dealer information from 242+ UK businesses, and insights drawn from years of hands-on experience in the gold market.
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Disclaimer: This guide provides general information about gold purity. Actual gold values depend on current market prices and dealer rates. For accurate valuations, use our calculator with live prices or consult a professional gold dealer. Information current as of February 2026.