Karat Calculator for Purity: How It Works

A karat calculator for purity converts the karat value of gold (e.g., 10K, 14K, 18K, 24K) into the percentage of pure gold by weight, and it can also work in reverse—turning a desired purity percentage into a karat value. You enter the karat number (or the percentage), and the calculator returns the corresponding purity fraction and often the gold content in grams for a given item weight. The math is simple: divide the karat value by 24, because 24 karats equals 100% pure gold.

Formula

The core formula is:

Purity (%) = (K / 24) × 100

Where:

• K = karat value of the gold (a number from 0 to 24, e.g., 14 for 14K)
• Purity (%) = the percentage of pure gold by weight in the alloy

To find the weight of pure gold in an item, multiply the total weight by the purity fraction:

Pure gold weight = Total weight × (K / 24)

For example, 18K gold is 18/24 = 0.75, or 75% pure gold. The remaining 25% is made up of other metals (copper, silver, zinc, nickel, etc.) that are added to increase hardness or change color.

Worked Example

Using the default values from the calculator above (18K, 10 grams):

1. Divide the karat value by 24: 18 / 24 = 0.75
2. Convert to a percentage: 0.75 × 100 = 75%
3. Pure gold weight: 10 g × 0.75 = 7.5 g
4. Alloy weight: 10 g − 7.5 g = 2.5 g

So a 10-gram 18K gold ring contains 7.5 grams of pure gold and 2.5 grams of other metals. The calculator shows the same results.

Why Karat Purity Matters

Karat is a measure of purity, not weight or density. It tells you how much of the metal is actually gold. Higher karat means more gold, which is softer and more valuable per gram. Lower karat gold is more durable and less expensive because it contains more alloy metals. Understanding karat purity is essential when buying jewelry, selling scrap gold, or comparing prices. A 14K gold chain and an 18K gold chain of the same weight will have different amounts of gold, so the price per gram should differ accordingly.

How to Read the Result

The purity percentage tells you the proportion of pure gold in the alloy. For example, 14K gold is 58.33% pure, 10K is 41.67%, and 24K is 99.9% (usually considered pure). The remaining percentage is the alloy. When you weigh an item, the pure gold weight is what you would use to estimate its gold value. Keep in mind that jewelry often has stones, clasps, or other non-gold parts that are not included in the gold weight—weigh only the gold parts for accurate calculations.

Common Mistakes and Misconceptions

• Confusing karat with carat: Karat (K) measures gold purity; carat (ct) measures gemstone weight. They are not interchangeable.
• Assuming 24K is 100%: In practice, 24K gold is usually 99.9% pure (999 parts per thousand), not exactly 100%.
• Mixing up weight and purity: A heavier 10K item may contain more gold than a lighter 18K item. Always multiply weight by purity to compare gold content.
• Using the wrong karat for stamped items: Some items are stamped with a number like 585 (which means 58.5% gold, or 14K). The calculator can convert that too: divide the number by 1000 to get purity, then multiply by 24 to get karat.

When to Use a Karat Calculator

Use this calculator when you need to:

• Estimate the pure gold content of a piece of jewelry before selling it.
• Compare the gold value of items with different karats.
• Check if a price per gram is fair by calculating the actual gold weight.
• Understand what a karat stamp means in terms of percentage.

It is also useful for hobbyists who make jewelry and need to know how much pure gold is in an alloy they created.

Limitations of the Formula

The formula assumes the karat value is accurate and that the item is solid gold throughout. It does not account for:

• Hollow areas or non-metal parts (e.g., gemstones, enamel, or cores made of other metals)
• Wear or plating that may change the surface composition
• Manufacturing tolerances—stamped karats can vary slightly (e.g., 14K might actually be 13.5K to 14.5K)

For scrap gold, professional assayers use fire assay or X-ray fluorescence to determine exact purity. A karat calculator gives a theoretical value based on the stamped karat, which is usually close enough for everyday estimation.

Frequently Asked Questions

What is the difference between 14K and 18K gold?

14K gold is 58.33% pure gold, while 18K is 75% pure. The rest is alloy metals. 18K has a richer yellow color and is more valuable per gram, but it is softer and more prone to scratches. 14K is harder and more durable, making it common for everyday rings and bracelets.

How do I convert a purity percentage to karat?

Divide the percentage by 100 to get a fraction, then multiply by 24. For example, 75% pure gold: (75/100) × 24 = 18K. Conversely, 22K gold is (22/24) × 100 = 91.67% pure.

Is 24K gold pure?

Technically, 24K gold is considered pure, but in practice it is usually 99.9% (999 fine) because 100% purity is not achievable in refined metals. Some coins are 99.99% (9999 fine), but jewelry is rarely 24K because it is too soft.

Why is 10K gold less expensive than 14K?

Because 10K contains only 41.67% gold, while 14K contains 58.33%. The lower gold content means less gold by weight, so the intrinsic value is lower. However, 10K is harder and more affordable, making it popular for budget-friendly jewelry.

Can I use this calculator for gold-plated items?

No. Gold-plated items have a thin layer of gold over a base metal. The karat stamp on plated items refers only to the plating, not the whole item. Weighing the entire piece would overestimate the gold content. This calculator works only for solid gold alloys.

What does the stamp “585” mean on jewelry?

It means the item is 58.5% pure gold, which is equivalent to 14K. European jewelry often uses parts-per-thousand stamps: 585 = 14K, 750 = 18K, 916 = 22K, 999 = 24K. To convert, divide the stamp by 1000, then multiply by 24 to get karat.

Related

• Gab Calculator Square Feet: How to Measure
• Google Calculator Square Feet: How to Use It
• Square Footage Calculator: Figure Sq Ft

in k | Karat value | K | 18 | 0
in w | Item weight | g | 10 | 0
out purity | Pure gold fraction | % | (k / 24) * 100 | 2
out gold_w | Pure gold weight | g | w * (k / 24) | 2
out alloy_w | Alloy weight | g | w - (w * (k / 24)) | 2