Gold Impurity Calculator (Grams) — Alloy Content & Refining Estimate
A gold impurity calculator works backwards from the karat purity stamp — given gross weight and karat, it returns the alloy (impurity) mass that will be lost during refining to pure 999.9 gold. For a 50 g 22K ring, the impurity mass is 50 × (2 ÷ 24) = 4.167 g; only 45.833 g pure gold will emerge from the refining process.
This calculation is the foundation of pre-refining valuation. Refiners pay you for pure gold yielded, not gross weight handed over. The impurity mass becomes by-product (typically copper, silver and zinc), which is sold separately and credited at a much lower rate.
When to use this calculator
- Pre-refining valuation — know exactly how much pure gold to expect back.
- Scrap dealer negotiation — verify the refiner's impurity claim against the karat stamp math.
- Alloy-by-product accounting — track separately for tax/inventory purposes.
Impurity, Alloy and the Gold Refining Process
Every gold piece below 24 karat contains alloy metals alongside the gold: 22K jewellery is 8.33% alloy by mass, 18K is 25% alloy, and 14K is over 41% alloy. The alloy composition varies by jewellery tradition — South Asian 22K typically uses copper and silver for a warmer yellow colour, European 18K white gold uses palladium or platinum for its silver-grey tone, and dental 14K may include indium or gallium for biocompatibility. The shared characteristic is that all these alloy metals will be separated during refining to pure 999.9 gold.
Knowing the impurity mass before submitting to a refiner is the single most important step in protecting your settlement. Most refiners pay only for the pure gold yielded after their process and deduct a refining fee on top. If you do not know your expected pure-gold yield in advance, you cannot verify whether the refiner's settlement figure is correct. The impurity calculator gives you that expected figure directly from the karat stamp and gross weight — no lab analysis required.
For the complementary calculation — how much alloy to add to a batch of pure gold to achieve a target karat — the gold impurity calculator (by karat or ratti) handles that direction. For a combined view of purity, impurity and money value in one screen, the gold analyzer integrates all three outputs without extra steps.
Impurity content math
Impurity (g) = Gross grams × (24 − karat) ÷ 24. Pure gold (g) = Gross grams × karat ÷ 24. Refining loss = up to 1% on top of stated alloy mass, depending on refiner process.
Step-by-step calculation
Example: 15 g of 18K gold
- 1
Calculate impurity ratio
ratio = (24 − karat) ÷ 24
(24 − 18) ÷ 24 = 6 ÷ 24 = 0.2500
- 2
Impurity mass (alloy)
alloy (g) = gross × ratio
15 × 0.25 = 3.750 g alloy
- 3
Pure gold mass
pure (g) = gross − alloy
15 − 3.750 = 11.250 g pure
- 4
Purity percentage
purity % = (1 − ratio) × 100
(1 − 0.25) × 100 = 75.00%
Sample conversions
| Input | Result |
|---|---|
| 10 g of 24K (999) | 0.01 g impurity (≈0.1%) |
| 10 g of 22K (916) | 0.833 g impurity (8.33%) |
| 10 g of 18K (750) | 2.500 g impurity (25%) |
| 10 g of 14K (583) | 4.167 g impurity (41.67%) |
Quick Reference — Karat to Impurity Content
| Karat | Impurity % |
|---|---|
| 24K (999) | 0.10% |
| 22K (916) | 8.33% |
| 21K (875) | 12.50% |
| 18K (750) | 25.00% |
| 14K (585) | 41.67% |
| 10K (417) | 58.33% |
| 9K (375) | 62.50% |
Impurity % = (24 − karat) ÷ 24 × 100. Add 0.5–1% for actual refining process loss on top.
Frequently asked questions
Why is "impurity" different from "refining loss"?
Impurity is the predictable alloy mass from the karat ratio (8.33% of 22K gross weight). Refining loss is the additional ~0.5–1% from oxidation, crucible spill and dust during the actual refining process — on top of the impurity mass. Both reduce your pure-gold return.
Are the alloy metals worth anything on resale?
Yes — copper, silver and palladium in the alloy are sold by the refiner as by-products. Most refiners credit a small sum for recoverable alloy metals in the settlement. It is typically 1–5% of the refining fee — meaningful on large scrap batches, negligible for a single piece.
What alloy metals are used in different karat gold jewellery?
Yellow 22K and 21K: copper + silver. White 18K: palladium, platinum or nickel. Rose gold (18K–22K): high copper content for the warm pink tone. Green gold (18K): silver-heavy alloy. Dental alloys (14K–18K): may include indium, gallium or platinum. The alloy recipe does not affect the karat purity calculation.
How do I calculate impurity percentage from a hallmark stamp alone?
Impurity % = (1,000 − fineness) ÷ 10. Example: 916 hallmark → (1,000 − 916) ÷ 10 = 8.4% alloy. Or: impurity % = (24 − karat) ÷ 24 × 100. Both give the same result.
What happens to gold alloy metals during the refining process?
Acid refining (aqua regia process) dissolves gold and platinum-group metals, while copper and silver precipitate or are separated chemically. The recovered copper and silver are sold as industrial by-products. The gold is precipitated, smelted and cast into new high-purity bars (typically 999.9 fine).
Does the impurity percentage affect the colour of gold jewellery?
Yes — colour depends on alloy composition, not purity alone. Yellow gold uses copper + silver; adding more copper gives rose/red gold; more silver gives green/pale gold. White gold uses palladium or platinum. Two pieces of the same karat can look completely different depending on which alloy metals are used.
Related calculators
These tools cover neighbouring steps in the same workflow — pair them with this one for a complete calculation.