Last updated · 2026-11-01
How we calculate
Where our gold and silver prices come from, the formulas and constants we use, and how photo hallmark reading works.
Live prices
Spot prices for gold (XAU) and silver (XAG) come from gold-api.com, in US dollars per troy ounce of pure metal. Our server fetches them and keeps each quote for five minutes, so every visitor in that window sees the same price. If the source cannot be reached we keep showing the last good quote and mark it as delayed, rather than showing nothing. You can always enter your own spot price in the calculator.
Spot is the wholesale price of pure metal. It is not what a shop pays you, and it is not a retail price for bullion.
The formula
Melt value = weight in grams × purity × spot price ÷ 31.1034768
31.1034768 is the number of grams in a troy ounce, the unit precious metals are priced in. Melt value is the worth of the pure metal inside a piece: the most a refiner could pay before its own costs.
Weights
| Unit | Grams |
|---|---|
| Troy ounce (ozt) | 31.1034768 |
| Pennyweight (dwt) | 1.55517384 (1/20 ozt) |
| Ounce, kitchen scale (oz) | 28.349523125 |
| Tola | 11.6638038 |
| Pound (lb) | 453.59237 |
Purities
Karat gold is parts of gold in 24, so we use the exact fraction: 14K is 14 ÷ 24 = 0.5833, not a rounded 0.585. Where a piece is stamped in parts per thousand, we use that number.
| Mark | Purity used |
|---|---|
| 24K / 999 | 0.999 |
| 22K / 916 | 0.9167 |
| 18K / 750 | 0.750 |
| 14K | 0.5833 |
| 585 | 0.585 |
| 10K | 0.4167 |
| 9K / 375 | 0.375 |
| 999 fine silver | 0.999 |
| 958 Britannia | 0.958 |
| 925 sterling | 0.925 |
| 900 coin | 0.900 |
| 835 | 0.835 |
| 800 | 0.800 |
Plated, gold-filled and rolled-gold pieces are valued at zero: the layer of precious metal is too thin for most buyers to pay for.
The likely cash offer
Buyers keep a margin for refining, assay and profit, so their offers land below melt value. We show 70–90% of melt as the usual range, which is the range calculator sites in this market quote. It is a rule of thumb, not a quote from any buyer, and you can change it in the calculator when you have a real offer to compare.
Reading a hallmark from a photo
When you use "read the mark from a photo":
- Your browser shrinks the photo and re-encodes it, which strips location and camera data.
- A vision model (Google Gemini) transcribes each stamp: the letters, numbers and symbols it can see. It is told not to judge purity, age or maker.
- Our own hallmark tables, not the model, turn those stamps into a metal and purity. Plating words such as EPNS, GF or 1/20 always win, because calling plate "sterling" is the costliest mistake.
- When the reading is unusual or the photo is unclear, a second model (Anthropic Claude) reads it again. If both agree, we show the result as read. If they disagree on purity for the same metal, we show the lower purity, so the estimate is never too high. If they disagree on the metal, we ask for a better photo.
In our first test on 15 public photos of real hallmarks with known answers, this method gave no wrong purity answers; it asked for a new photo on 3 of them. Fifteen photos is a small sample, so treat a reading as a strong hint and check the stamp yourself.
Photos are read once and not stored.
What this is not
Mark & Melt gives estimates for selling scrap. It is not an appraisal, an authentication or an offer to buy, and we do not buy or sell metal. For insurance, estates or high-value pieces, use a qualified appraiser.