Gold Purity Calculator: Karat, Fineness and Fine Gold

A gold purity calculator estimates how much of an item’s total weight is pure gold, using its karat rating or gold percentage. Enter the item’s weight and purity to get its estimated fine-gold content; the result is a mathematical estimate, not a test of the metal.

Formula

Fine gold content = total item weight × gold purity as a fraction. In symbols, F = W × P / 100, where F is fine gold content in grams, W is the item’s total weight in grams, and P is gold purity as a percentage. The other-metal content is A = W − F, also in grams. The calculator reports both results to two decimal places.

For a karat value, convert karats to a percentage with P = K / 24 × 100, where K is the karat rating. For example, 18-karat gold is 18/24, or 75% gold. The calculator takes the percentage directly, so convert a karat mark before entering it.

Worked Example

Using the defaults—10 g of item weight and 75% purity:

• Convert the percentage to a fraction: 75 / 100 = 0.75.
• Multiply the weight by that fraction: 10 g × 0.75 = 7.5 g of fine gold.
• Subtract the fine gold from the total: 10 g − 7.5 g = 2.5 g of other metals.

Rounded to two decimal places, the calculator returns 7.50 g of fine gold and 2.50 g of other metals.

What Gold Purity Means

Gold purity describes the proportion of gold in a metal alloy. Pure gold is defined as 24 karats, while lower-karat jewelry contains gold mixed with other metals. These additions can change hardness, color, and working properties; the calculator only estimates their combined weight and does not identify which metals are present.

Two common ways of expressing purity are karats and fineness. Karats divide the alloy into 24 parts. Fineness states the number of parts of gold per 1,000 parts of alloy. Thus, 18-karat gold corresponds to 750 fineness, or 75% gold. A mark such as “585” conventionally indicates 585 parts per thousand, equivalent to 58.5% gold. Stamps and marking practices vary, so a marking is a useful clue, not proof that an item has that composition.

How to Use the Calculator

1. Weigh the item and enter its mass in grams. If your scale reads another unit, convert the weight first.
2. Find the stated purity, if known. Enter a percentage such as 75 for 75%, not 0.75.
3. If the item is marked in karats, convert using karats ÷ 24 × 100. For fineness, divide the number by 10 to get a percentage: 750 becomes 75%.
4. Read the fine-gold result as the estimated mass of gold, and the other-metal result as the remaining mass under the entered assumptions.

For example, a 12 g item entered at 58.5% purity contains an estimated 7.02 g of fine gold: 12 × 0.585. The remaining 4.98 g is alloy or other material included in the weight. This calculation does not determine the item’s market value; that would require current pricing and other information not included here.

Units, Scope and Important Limits

The calculator uses grams because jewelry and precious-metal measurements commonly use metric mass units. It can also be used with ounces if the same weight unit is used consistently: enter the item’s weight in ordinary avoirdupois ounces, and the answer will be in those ounces. For precious metals, a troy ounce is different: one troy ounce is about 31.1035 grams, while one avoirdupois ounce is about 28.3495 grams. Convert carefully, and do not label one type of ounce as the other.

The formula assumes the entered percentage applies uniformly to the entire weight. It cannot account for stones, watch mechanisms, non-gold components, solder, plating, hollow spaces, or dirt unless those are removed from the weight or otherwise accounted for. A gold-plated object, for example, cannot be accurately represented by entering a karat mark for the thin surface layer: most of its weight may be a different base metal.

Rounding also matters. A scale’s reading and a purity estimate both have limited precision, so a result such as 7.50 g should not be treated as proof of exactly that amount. The calculation does not assay the item. A reliable composition determination may require appropriate testing by a qualified professional or laboratory, especially when the result matters for a transaction.

Common Mistakes and Misconceptions

• Confusing karats with carats: Karat measures gold purity; carat is a unit of gemstone mass. They are not interchangeable.
• Entering a karat number as a percentage: Entering 18 means 18%, not 18-karat. Convert 18 karats to 75% first.
• Assuming a stamp guarantees purity: A hallmark or stamp can be informative, but this calculator cannot verify it and markings may be absent or misleading.
• Counting the whole object as gold jewelry: Stones, clasps, springs, and other parts can add weight without adding gold. The formula uses the weight you provide as if it all has the stated purity.
• Assuming “other metal” means one specific alloy: The result is only the remainder. It does not reveal whether that material is copper, silver, nickel, or something else.

Frequently Asked Questions

How do I calculate pure gold from karats?
Divide the karat number by 24, then multiply by the item’s weight. For 10 g of 14-karat gold: 10 × 14/24 = about 5.83 g of fine gold, assuming the full item weight has that purity.

What percentage is 18-karat gold?
18-karat gold is 18/24, or 75% gold. Its corresponding fineness is 750 parts per thousand.

What does 585 mean on gold?
It usually denotes 585 parts gold per 1,000 parts alloy, equivalent to 58.5% purity and approximately 14 karats. The mark alone does not authenticate an item.

Does this calculator tell me what my gold is worth?
No. It estimates fine-gold mass from the weight and purity you enter. Valuation also depends on factors such as the applicable metal price, the item’s condition, and how it is assessed.

Can I calculate purity without knowing the karat?
You can calculate the fine-gold content only if you have a usable purity percentage or another reliable purity measure. Weight alone cannot reveal composition; it needs to be tested or otherwise established.

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in weight | Item weight | g | 10 | 0
in purity | Gold purity | % | 75 | 0
out fine | Fine gold content | g | weight * purity / 100 | 2
out alloy | Other metal content | g | weight - fine | 2