Find Purity From Weight
Total weight and non-gold weight determine the gold percentage.
Purity % = Gold Weight ÷ Total Weight × 100
This tool derives gold content from the supplied weight difference. It does not chemically test the material.
How Gold Purity From Weight Is Calculated
If the total material weight and the non-gold weight are known, the weight of gold can be calculated by subtraction.
The gold percentage is then calculated from the gold weight divided by the total weight.
Example: 20 Grams Total, 5 Grams Non-Gold
An item weighs 20 grams in total and the non-gold portion weighs 5 grams.
Purity = 15 ÷ 20 × 100 = 75%
75% corresponds mathematically to 18K and 750 fineness.
Example: 50 Grams Total, 8 Grams Non-Gold
Purity = 42 ÷ 50 × 100 = 84%
Karat = 84 ÷ 100 × 24 = 20.16K
Fineness = 840
This illustrates that the result does not have to be one of the standard commercial karat grades.
Gold Purity Results From Weight Ratios
| Total Weight | Non-Gold | Gold Weight | Purity | Approx. Karat |
|---|---|---|---|---|
| 10 g | 0 g | 10 g | 100% | 24K |
| 10 g | 0.8333 g | 9.1667 g | 91.67% | 22K |
| 10 g | 2.5 g | 7.5 g | 75% | 18K |
| 10 g | 4.1667 g | 5.8333 g | 58.33% | 14K |
| 10 g | 5.8333 g | 4.1667 g | 41.67% | 10K |
| 10 g | 6.25 g | 3.75 g | 37.5% | 9K |
Why This Calculator Is Different
This calculator begins with two physical weight measurements: total material weight and non-gold weight. It derives the gold portion instead of asking the user to enter a known fine-gold weight.
Jewellery and Non-Gold Components
For jewellery, non-gold weight can theoretically include gemstones, coatings, other metal components or other materials. The calculation is only meaningful if the non-gold measurement has been determined correctly.
A scale alone cannot normally tell you how much of a jewellery item's total weight is gold versus gemstones or other components.
Weight-Based Purity vs Laboratory Testing
A weight ratio can calculate a mathematical purity if the gold and non-gold weights are accurately known. It is not equivalent to chemical or assay testing of an unknown piece of metal.
For commercial or authentication decisions, appropriate testing should be used when composition is uncertain.
Important Limitations
The result depends entirely on the supplied total and non-gold weights. If those measurements include incorrect components, the calculated purity will also be incorrect.
This calculator does not identify the exact alloy metals or determine market value.
Frequently Asked Questions
How do you calculate gold purity from weight?
Subtract non-gold weight from total weight to obtain gold weight, then divide gold weight by total weight and multiply by 100.
What if I know total weight and non-gold weight but not gold purity?
Enter both values and the calculator derives gold weight, percentage, karat and fineness.
If an item weighs 20 grams and 5 grams is non-gold, what is the purity?
The gold portion is 15 grams. 15 ÷ 20 = 75%, corresponding mathematically to 18K and 750 fineness.
Can this calculator calculate jewellery purity with gemstones?
It can calculate a mathematical ratio, but the non-gold input must correctly account for all material that should be excluded from the gold alloy.
Can the calculator identify the exact karat of unknown gold?
It calculates an equivalent karat from the supplied weight ratio. It does not independently test the metal.
Does this calculator measure gold value?
No. It calculates weight-based purity only.