In the Indian precious metals trade, whether you are running a jewellery showroom, trading bullion in Zaveri Bazaar, or melting old ornaments at a refinery intake counter, tunch (purity percentage) is the universal benchmark. Every commercial transaction—from a 5-gram ring exchange to a 5-kilogram TT bar deal—depends on converting gross ornament weight into pure 999.0 fine weight. Here is how purity conversion works, the exact mathematical formulas used across Indian mandis, and how you can calculate fine weight instantly with our free online tool.
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What is Tunch in the Indian Sarafa Market?
In Indian bullion and jewellery terminology, tunch (टंच) refers to the percentage of pure gold or silver contained within a given alloy or ornament:
- A tunch of 91.60% represents 22-Karat hallmark gold (916 purity).
- A tunch of 75.00% represents 18-Karat gold (750 purity).
- A tunch of 99.50% represents standard bullion bars (995 fineness).
- A tunch of 99.90% represents pure 24-Karat investment gold (999 fineness).
When a jeweller accepts old scrap gold, an XRF spectrometer reading or traditional touchstone / fire assay report yields a tunch number (e.g., 84.75%). You must calculate the exact pure metal yielded before determining cash payout or issuing a purity exchange voucher.
The Fine Weight Formula Used by Every Sarraf
Calculating fine weight requires only three steps:
1. Fine Weight Formula:
Fine Weight (g) = (Gross Weight in Grams × Purity Tunch %) ÷ 100
2. 999.0 Standard Pure Benchmark:
999 Fine Weight (g) = Fine Weight ÷ 0.999
3. Metal Value at Mandi Spot Rate:
Total Value (₹) = (Fine Weight in Grams ÷ 10) × 10g Gold Spot Rate (₹)
Practical Example:
Suppose a customer brings 150.000 grams of old ancestral jewellery. The XRF test determines a purity of 82.40%:
Gross Weight = 150.000 g
Purity = 82.40%
Fine Gold Content = (150.000 × 82.40) ÷ 100 = 123.600 g Fine Gold
If today's 24K spot rate is ₹74,500 per 10 grams:
Estimated Metal Value = (123.600 ÷ 10) × ₹74,500 = ₹9,20,820
Karat to Fineness & Purity Reference Table
Keep this standard conversion chart handy for daily showroom and counter operations:
| Karat | Hallmark Fineness | Purity Tunch % | Common Application |
|---|---|---|---|
| 24K | 999 | 99.9% | Minted coins, TT bars, bank bullion |
| 23K | 958 | 95.8% | Traditional wedding jewellery in south India |
| 22K | 916 | 91.6% | Standard Indian retail gold jewellery |
| 20K | 833 | 83.3% | Antique jewellery, temple ornaments |
| 18K | 750 | 75.0% | Diamond-studded jewellery & luxury watches |
| 14K | 585 | 58.5% | Modern lightweight and export jewellery |
Why Manual Desk Calculators Cause Costly Leaks
In high-volume trading, manual calculation poses significant operational hazards:
- Rounding Discrepancies: Rounding 3-decimal places prematurely across 50 daily transactions can easily shift 5 to 10 grams of fine gold out of balance by month's end.
- Customer Trust & Transparency: When jewellers show calculations on a phone or paper scrap, customers may question the deduction. Pointing them to a verified, clear digital calculator establishes instant credibility.
- Silver Calculations: While gold is quoted per 10 grams in India, silver is quoted per kilogram. Mixing up the unit divisor leads to massive calculation blunders.
Beyond Single Calculations: Full Operating Systems
While our Free Purity Calculator is perfect for quick desk calculations, active trading businesses require automated ledger integration:
- For Bullion Wholesalers & Sarrafs: Bullion Master automatically computes fine metal weight for every trade, manages unhedged Net Daily Positions (NDP), and tracks badla purity swaps.
- For Refinery Owners: Refinery Master links raw intake, melting loss, assay tunch, and yield recovery directly into customer running fine metal ledgers.