Gold Purity Calculator - Check Karat, Assay % & Pure Gold Content

The Math Behind Every Hallmark Stamp

Gold is never sold as a single, uniform product. A 22K ring and an 18K bracelet sitting side by side in a jeweller's display case contain fundamentally different amounts of gold - even if they weigh exactly the same. The number stamped on each piece is a fineness indicator, and translating it correctly is what separates an informed buyer or seller from one who accepts whatever price they're offered.

GoldPrice.Live built this gold purity calculator to make that translation instant and exact. Enter any weight, select the karat grade, and the tool returns the assay purity percentage, the corresponding fineness stamp, and the pure gold content in grams - all against the live XAU/USD spot rate.

How the Karat System Actually Works

The karat scale runs from 0 to 24, where 24 represents pure gold. Each karat unit equals one part out of 24 - so the formula for any grade is straightforward:

Purity % = (Karat ÷ 24) × 100

That single formula underpins every conversion this calculator makes:

1. 24K → 99.99% pure → fineness stamp 999
2. 22K → 91.67% pure → fineness stamp 916
3. 21K → 87.5% pure → fineness stamp 875
4. 18K → 75.00% pure → fineness stamp 750
5. 14K → 58.33% pure → fineness stamp 585
6. 10K → 41.67% pure → fineness stamp 417

The fineness stamp - the three-digit number - is the same purity expressed in parts per thousand instead of parts per 24. It's the format used on hallmarked jewellery across Europe, the Middle East, and South and Southeast Asia. When you see 750 stamped on a ring, it's the same piece of information as 18K, written in a different notation.

What "Assay Purity" Means - And When It Matters

An assay is a formal test of metal composition. The assay purity percentage shown by this calculator is the theoretical gold content implied by the karat grade - for example, 75.00% for 18K. In practice, real-world gold alloys manufactured to a stated karat are required to meet minimum fineness thresholds, not exact values. An 18K piece can legally test anywhere from 74.9% to slightly above 75%, depending on the jurisdiction and assay tolerance.

This matters most in three situations:

1. Before selling to a refiner. Refiners assay your gold before paying. If a piece tests marginally below its stated karat, the payout reflects the actual assay result, not the hallmark. Running this calculator gives you the theoretical maximum for what you're holding - the assay result from a refiner may vary slightly.
2. When buying unmarked jewellery. Vintage and antique pieces, particularly those made before hallmarking was mandatory in a given market, may carry no stamp at all. An acid test or XRF scan gives the actual fineness; this calculator then converts that reading to karat and shows the pure gold content.
3. When verifying a purchase. If a piece is stamped 916 but tests at 875, that's a meaningful discrepancy - the fineness column in this calculator makes the expected value immediately clear.

For the live market value of that pure gold content once you've established fineness, the live gold price feed updates continuously with the XAU/USD spot rate.

Reading a Hallmark: What the Stamps Tell You

Hallmarks vary by country but follow consistent logic. Here's what you're likely to encounter:

1. 999 / 999.9 - Investment-grade fine gold. Coins, bars, and bullion products. Rarely used in jewellery because pure gold is too soft for settings.
2. 916 - 22K. The dominant standard for jewellery across India, the Middle East, and much of Southeast Asia. High purity with enough alloy for durability.
3. 875 - 21K. Common in the Gulf states and parts of Eastern Europe. Less common in Western markets.
4. 750 - 18K. The preferred grade for fine jewellery in Europe and North America. Strong enough for gemstone settings, high enough purity to hold its colour.
5. 585 - 14K. The most common grade in the United States for everyday jewellery. Durable, affordable, meaningful gold content.
6. 417 - 10K. The legal minimum for gold jewellery in the US. Predominantly alloy by weight, but still carries genuine gold content.

If you're looking at how karat grade directly affects the current price per gram, the 24K gold price and 18K gold price pages show live per-gram rates for specific grades with full market context.

Pure Gold Content: Why Total Weight Is Misleading

A 10-gram 14K bracelet does not contain 10 grams of gold. It contains 5.83 grams of pure gold and 4.17 grams of alloy - typically copper, silver, zinc, or a combination. When a dealer quotes a price for that bracelet as scrap, they're pricing the 5.83 grams of gold, not the 10 grams of total weight.

This calculator isolates the pure gold content automatically. Enter the total weight and karat, and the pure gold output in grams is what your metal is actually worth at spot. That figure is also the starting point for scrap valuations - if you're planning to sell, the scrap gold calculator takes the pure content figure and applies a dealer payout percentage to show a realistic offer range.

For deeper context on how the precious metals market is moving while you assess your holdings, the gold market commentary covers current price drivers and macro trends.

If you have a specific piece you're trying to identify or a hallmark you can't decode, get in touch and the team can help.