Gold Impurity Calculator — Alloy Mass & Refining Loss Estimator
The gold impurity calculator works out the alloy (impurity) content of any gold piece given its karat purity and gross weight. The impurity mass is what gets discarded during refining to pure 999.9 gold — meaning that mass is not paid back to you in the bullion-equivalent yield.
Use this calculator before sending pieces to a refiner so you know exactly how much pure gold to expect back. Pick your preferred input unit — grams or tola — and the tool returns impurity mass in matching units.
When to use this calculator
- Pre-refining valuation
- Refiner quote verification
- Insurance appraisal
Understanding Gold Impurity Before Sending to a Refiner
Every gram of alloy in a gold piece reduces the yield when refining to pure 999.9 gold. Before handing jewellery to a refiner, knowing the theoretical impurity mass — derived from the karat stamp — sets the baseline for evaluating the refiner's assay report and expected return. A 22K piece at 8.33% alloy by weight gives a clear benchmark; if the refiner's report shows 10% alloy, you have grounds to request an independent XRF or acid test.
The impurity calculator uses the karat fraction directly: alloy mass = gross × (24 − karat) ÷ 24. This is identical math to the karat purity calculator's impurity output, surfaced as a standalone tool for clarity in refining conversations. Both gram and tola input variants are available to match the unit on the piece's original purchase invoice.
For a complete combined view showing pure gold, alloy mass, fineness number and live money value all at once, the gold analyzer is the more efficient tool. For quick per-karat reference without entering a specific weight, the quick-reference table below is sufficient.
Impurity content math
Impurity = Gross × (24 − karat) ÷ 24. Same formula works for any input unit (grams, tola, troy oz). For 22K: impurity = 8.33% of gross weight.
Step-by-step calculation
Example: 20 g of 18K gold
- 1
Calculate impurity ratio
ratio = (24 − karat) ÷ 24
(24 − 18) ÷ 24 = 0.2500 (25%)
- 2
Alloy (impurity) mass
alloy (g) = gross × ratio
20 × 0.25 = 5.000 g alloy
- 3
Pure gold mass
pure (g) = gross × (1 − ratio)
20 × 0.75 = 15.000 g pure
- 4
Hallmark fineness
fineness = karat ÷ 24 × 1000
18 ÷ 24 × 1000 = 750 fine
Sample conversions
| Input | Result |
|---|---|
| 10 g 22K | 0.833 g alloy (8.33%) |
| 1 tola 18K | 0.25 tola alloy (3 masha) |
Quick Reference — Impurity % and Mass per 10 g by Karat
| Karat | Impurity % |
|---|---|
| 24K (999) | 0.1% |
| 22K (916) | 8.33% |
| 21K (875) | 12.5% |
| 18K (750) | 25.0% |
| 14K (585) | 41.67% |
| 10K (417) | 58.33% |
Impurity % = (24 − karat) ÷ 24 × 100. Add 0.5–1% for actual refining process loss.
Frequently asked questions
What metals make up the impurity in 22K gold?
Typically a copper-silver mix for yellow 22K. Rose 22K is heavier on copper for the warm tone; pale/lime 22K uses more silver. Some makers add zinc in small amounts. The exact recipe is the jeweller's proprietary alloy formula.
How do I calculate the impurity in my gold jewellery by weight?
Impurity (g) = Gross weight × (24 − karat) ÷ 24. Example: 15 g of 18K = 15 × (6/24) = 3.75 g alloy. Enter the gross weight and karat above and the calculator returns impurity instantly.
Is the impurity calculator appropriate for scrap gold valuation?
Yes — it gives the theoretical alloy mass from the karat stamp. Combined with the live money value (pure-gold grams × spot price), it shows what a refiner should return. Subtract refining fees (1–3%) and process loss (0.5–1%) for the net expected payout.
Can I use this to verify a refiner's assay report?
Yes — compare the refiner's reported impurity percentage to this calculator's output. If the refiner's figure is significantly higher than the karat-stamp prediction, either the karat stamp is inaccurate or the refiner's assay found lower purity. Request an independent acid or XRF test to resolve the discrepancy.
What is the impurity content of a 10K gold piece?
10K = 10/24 = 41.67% pure gold, so 58.33% is alloy/impurity. For a 10 g piece: 5.833 g is pure gold and 4.167 g is alloy. 10K is the minimum legal karat for gold jewellery in the US; it is rarely seen in South Asian markets.
Related calculators
These tools cover neighbouring steps in the same workflow — pair them with this one for a complete calculation.