Understanding Gold Purity
Gold purity determines how much actual gold is present in your sample. Jewellery and scrap gold are rarely 100% pure; they're alloyed with copper, silver, or other metals for durability and cost. Two measurement systems express this:
- Karats (K): A scale of 1–24, where 24K is pure gold. Each karat represents 1/24th of the total weight. 18K gold contains 18 parts pure gold out of 24, so 18/24 = 75% pure.
- Percentage purity: Expressed as 100% maximum. 9K gold is 9/24 = 37.5% pure; 22K is 91.7% pure.
Common jewellery grades include 24K (pure), 22K (91.67%), 18K (75%), 14K (58.3%), and 10K (41.7%). Lower-karat alloys are stronger and cheaper but contain less precious metal per gram.
Scrap Gold Value Formula
The calculation involves three steps: convert purity to a decimal, find the pure gold weight, then multiply by the current spot price.
Purity (decimal) = Karat ÷ 24
Pure Gold Weight = Mass (grams) × Purity (decimal)
Value = Pure Gold Weight (troy oz) × Price per Troy Ounce
Mass— Total weight of your scrap gold sample in grams or troy ouncesKarat— Purity rating from 1 to 24; divide by 24 to get the decimal purityPurity (decimal)— The fraction of pure gold in your sample; 18K = 0.75, 14K = 0.583, etc.Pure Gold Weight— Mass multiplied by purity—the actual amount of pure gold you ownPrice per Troy Ounce— Current spot price; updated daily based on commodity marketsValue— Total resale value of your scrap gold at current market rates
Worked Example: Two Batches of Scrap Jewellery
Imagine you inherit a collection of old gold jewellery. After sorting, you have two batches: one marked 18K and another marked 9K. You weigh them with a digital scale.
- Batch 1 (18K): 120 grams. Purity = 18/24 = 0.75 (75% pure). Pure gold weight = 120 × 0.75 = 90 grams = 2.89 troy ounces.
- Batch 2 (9K): 350 grams. Purity = 9/24 = 0.375 (37.5% pure). Pure gold weight = 350 × 0.375 = 131.25 grams = 4.22 troy ounces.
If spot gold is £50 per troy ounce, Batch 1 is worth 2.89 × £50 = £144.50, and Batch 2 is worth 4.22 × £50 = £211. Total: £355.50. Actual dealer offers may be 5–10% lower after refining costs.
Key Caveats When Selling Scrap Gold
Spot price doesn't equal what you'll receive; several real-world factors reduce your payout.
- Dealer margins and refining fees — Buyers don't pay spot price directly. They deduct 5–15% for refining, testing, and profit. A dealer might pay 85% of calculated value. Always get quotes from multiple buyers.
- Purity verification requires assay — If your gold isn't hallmarked or tested, a dealer will assay it at their cost or discount heavily to cover risk. Bring paperwork or certification if available to reduce suspicion.
- Weight loss during refining — Solder, plating, and contaminants add weight but aren't pure gold. Expect 0.5–2% loss when melted. Old jewellery with intricate joints loses more than clean scrap.
- Spot price volatility — Gold prices move daily and can swing £5–£10 per ounce in hours. Lock in a quote immediately if you're selling; don't wait for prices to rise.
Choosing Between Karat and Percentage Input
This calculator accepts both karat markings and percentage purity. Jewellery typically uses karats (stamped inside); industrial or raw gold stock may use percentages. The conversion is simple: divide karats by 24 to get the decimal form.
- 18K = 18/24 = 0.75
- 14K = 14/24 ≈ 0.583
- 10K = 10/24 ≈ 0.417
- 22K = 22/24 ≈ 0.917
If you know the percentage (e.g. 91.7%), enter it directly. The calculator will compute pure weight identically. Troy ounces (31.1 grams) are standard in precious metals trading, so always confirm whether your spot price quote uses troy ounces or standard ounces.