Gold Purity Percentage: Exact Karat Values

This calculator works out the exact gold purity percentage from a karat value, and it also converts a purity percentage back into the karat scale. You enter the karat number (for example 18, 14, or 22) or the percentage (like 75%), and you get the precise fineness in both systems, plus the millesimal fineness (parts per thousand) that jewelers use.

Formula

The purity of gold is defined as the ratio of pure gold to the total alloy weight, expressed in parts per 24. The conversion formulas are:

• Karat to percentage: Purity (%) = (Karat / 24) × 100
• Percentage to karat: Karat = (Purity % / 100) × 24
• Millesimal fineness: Fineness (‰) = (Karat / 24) × 1000

Where Karat is the traditional purity unit (24 = pure gold), Purity is the percentage of gold by mass, and Millesimal fineness is the parts per thousand. All values are based on mass, not volume.

Worked Example

Using the default values from the calculator (18 karat and 75%):

1. Convert 18 karat to percentage: (18 / 24) × 100 = 0.75 × 100 = 75.00%
2. Convert 75% to karat: (75 / 100) × 24 = 0.75 × 24 = 18.00 karat
3. Convert 18 karat to millesimal fineness: (18 / 24) × 1000 = 0.75 × 1000 = 750‰

The calculator returns 75.00% for the first output, 18.00 karat for the second, and 750‰ for the third, matching the steps above.

Exact Purity Percentages for Common Karats

Because karat is a ratio out of 24, the percentage is always a fixed decimal. Here are the exact values for the most common marks:

• 24 karat: 100.00% pure gold (999‰ or higher)
• 22 karat: 91.67% (916.7‰) – common in gold coins and some jewelry
• 18 karat: 75.00% (750‰) – standard for fine jewelry in Europe and Asia
• 14 karat: 58.33% (583.3‰) – standard in the US for durable jewelry
• 10 karat: 41.67% (416.7‰) – the minimum karat that can legally be called gold in the US
• 9 karat: 37.50% (375‰) – common in the UK and Australia

Note that 14 karat is often stamped as 585 (instead of 583) because the industry rounds to the nearest thousandth. Similarly, 22 karat is sometimes stamped 917. These are the same purity, just different rounding conventions.

How to Read a Hallmark or Stamp

Gold jewelry is usually stamped with a number that tells you its purity. You might see:

• Karat marks: 24K, 18K, 14K, 10K, etc.
• Percentage marks: 750, 585, 375 (these are millesimal fineness, not percentages)
• Decimal marks: 0.750, 0.585, 0.375 (fraction of pure gold)

If you see a three-digit number like 750, it means 750 parts per thousand – exactly 75% gold. A stamp of 585 means 58.5% gold, which is the rounded version of 14 karat. Always check the stamp, but remember that stamps can be inaccurate or worn, so a professional test (acid or X-ray fluorescence) is the only way to be certain.

Why Purity Percentages Are Not Always Exact

In practice, the exact percentage can vary slightly from the theoretical value. Most countries allow a tolerance (often ±0.5% to ±1%) for mass-produced items. For example, a ring stamped 18K might actually be 74.8% or 75.2% gold and still be legal. Also, some manufacturers intentionally use a slightly higher purity (like 750.5‰) to ensure they stay above the minimum. This is why the calculator gives the theoretical percentage, but the real item may differ by a fraction of a percent.

Another reason for variation is that some regions use different standards. For instance, “14K” in the US is 58.33%, but in Italy it is often 58.5% (585‰). The difference is tiny but can matter for high-value transactions.

Common Mistakes and Misconceptions

• Confusing karat with carat: Karat (K) measures gold purity; carat (ct) measures gemstone weight. They are unrelated.
• Assuming 18K = 75% gold by volume: It is by mass, not volume. Gold is denser than alloy metals, so the volume percentage is lower.
• Thinking 24K is always 100%: Pure gold is 99.99% (999.9‰) because it is nearly impossible to refine to 100.00%. It is still called 24K.
• Rounding too early: If you round 14/24 to 0.58 before multiplying, you get 58% instead of 58.33%. Keep the fraction until the end.
• Using the stamp as absolute truth: Stamps can be fake or worn. Always test if the value is significant.

How to Choose the Right Purity

The best karat for you depends on how you plan to use the gold:

• For daily wear jewelry (rings, bracelets): 14K or 10K is more durable because the alloy metals add strength. 18K is softer and scratches more easily.
• For investment or coins: 24K or 22K is preferred because it has a higher gold content. Coins are often 22K (91.67%) to balance purity with durability.
• For fine jewelry that is not worn daily: 18K or 22K offers a richer color and higher value, but requires more care.
• For hypoallergenic concerns: Higher karat (18K or above) contains less nickel and other allergens, but even 24K can cause reactions in rare cases due to other impurities.

Remember that higher purity means a higher price per gram, but also a softer metal. There is no “best” – only what fits your needs.

Frequently Asked Questions

What is the exact percentage of 18K gold?
18K gold is exactly 75.00% pure gold. The remaining 25% is made up of other metals like copper, silver, or zinc.

Is 14K gold 58.3% or 58.5%?
14K is mathematically 58.333…% (14/24). Many countries round this to 585‰ (58.5%) for stamping, so both are accepted. The actual gold content is the same.

How do I convert karat to percentage without a calculator?
Divide the karat number by 24, then multiply by 100. For example, 10K: 10 ÷ 24 = 0.4167, then × 100 = 41.67%.

What does 999 mean on gold?
999 means 999 parts per thousand, or 99.9% pure gold. This is commonly called “three nines fine” and is often used for bullion.

Can gold purity be over 100%?
No, 100% is the theoretical maximum. In practice, the highest achievable is about 99.999% (five nines) in laboratory conditions, but commercial gold is usually 99.99%.

Why is 24K gold not used for most jewelry?
24K gold is too soft for everyday wear – it bends, scratches, and deforms easily. Alloy metals are added to increase hardness and durability, which is why lower karats are common for rings and bracelets.

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in k | Karat value | karat | 18 | 0
in p | Purity percentage | % | 75 | 0
out pct | Gold purity | % | k / 24 * 100 | 2
out karat | Karat equivalent | karat | p / 100 * 24 | 2
out mille | Millesimal fineness | ‰ | k / 24 * 1000 | 0