Gold purity tells you how much of an alloy is gold by mass. Karat divides the alloy into 24 parts. Millesimal fineness divides it into 1,000 parts. Therefore 18K gold is 18/24 = 75% gold = 750 fine.
Karat versus fineness
Karat—written K, Kt or in another locally used form—describes gold in twenty-four parts. Millesimal fineness expresses gold in parts per thousand. Percentage expresses it per hundred. The three systems describe the same underlying ratio.
gold percentage = (karat ÷ 24) × 100 millesimal fineness = (karat ÷ 24) × 1,000 karat = gold percentage × 0.24In calculation, convert the percentage to a decimal: 75% becomes 0.750. Multiplying a 20 gram gold-bearing alloy by 0.750 gives 15 grams of nominal fine gold.
Gold karat and fineness chart
| Karat | Exact karat fraction | Nominal percentage | Related millesimal expression | Fine gold in 10 g of alloy |
|---|---|---|---|---|
| 24K | 24/24 | 100% in karat arithmetic | Common refined products may state 999 or 999.9 | Depends on stated fineness; 9.999 g at 999.9 |
| 23K | 23/24 | 95.83% | About 958 | About 9.583 g |
| 22K | 22/24 | 91.67% | About 916 or 917 | About 9.167 g |
| 21K | 21/24 | 87.50% | 875 | 8.750 g |
| 20K | 20/24 | 83.33% | About 833 | About 8.333 g |
| 18K | 18/24 | 75.00% | 750 | 7.500 g |
| 15K | 15/24 | 62.50% | 625 | 6.250 g |
| 14K | 14/24 | 58.33% | Often 585; arithmetic gives about 583.3 | About 5.833 g |
| 12K | 12/24 | 50.00% | 500 | 5.000 g |
| 10K | 10/24 | 41.67% | About 417 | About 4.167 g |
| 9K | 9/24 | 37.50% | 375 | 3.750 g |
| 8K | 8/24 | 33.33% | About 333 | About 3.333 g |
Why 14K and 585 do not look perfectly identical: 14/24 equals 0.58333 recurring, while 585 states 0.585. Commercial standards and permitted descriptions are legal and historical conventions, not merely rounded calculator outputs.
Is 24K the same as 999.9?
"24 karat" is treated as pure for trade description, while a refined bar marked 999.9 makes a more specific nominal claim of 99.99% gold. Do not silently replace a stated assay or fineness with a broader karat label. When valuing a product, use the product's documented or tested fineness.
What a gold mark can and cannot tell you
A fineness mark such as 750 states a claim that the gold-bearing alloy is nominally 750 parts gold per thousand. A hallmarking system can add a chain of responsibility involving a maker or sponsor and an assay office. Such systems developed because a buyer could not easily verify a finished object without damaging it.
Marks reduce uncertainty; they do not determine the entire sale price. A mark can be false, worn, transposed, inconsistent with the object's construction or valid under a standard different from the one a reader assumes. It also cannot establish that every gram on the scale is gold-bearing alloy.
- Read the whole group: inspect all symbols, numbers and letters rather than one isolated stamp.
- Consider the object: clasps, springs, stones, solder and later repairs can have different compositions.
- Preserve provenance: invoices, maker information and prior assay records can support classification.
- Verify when needed: the higher the financial consequence, the stronger the evidence should be.
Solid alloy versus a gold-colored surface
A purity calculation assumes that the entered gold-bearing weight has a reasonably uniform composition. A surface coating does not justify applying its apparent karat to the entire object. Plating can also mislead methods that read only or mainly near the surface.
Color alone is not an assay. Alloying additions change color and working properties, while surface treatments, wear and lighting alter appearance. The commercial question is not simply "Does it look gold?" but "What portion of the measured object is gold, and how was that established?"
How gold purity is tested
| Approach | Use | Important limit |
|---|---|---|
| Visual inspection and marks | Fast screening and classification | A stamp or appearance can be false, altered or misunderstood |
| Touchstone and acid | Estimates surface karat by comparison or reaction | Primarily examines the tested area and can be affected by surface composition |
| Electronic conductivity instruments | Non-destructive screening | Geometry, calibration and unusual construction can affect interpretation |
| X-ray fluorescence | Non-destructive elemental reading near the surface | Plating and surface composition can misrepresent the interior |
| Fire assay | Highly accurate analysis of a representative sample | Destructive and usually disproportionate for an intact small retail item |
Buyers combine methods according to item, lot size and risk. For large or mixed lots, representative sampling matters as much as instrument precision.
How purity changes melt value
If two objects have the same net gold-bearing weight, the higher-fineness object contains more fine gold. At the same reference price, 20 grams of nominal 18K alloy contains 15 grams of fine gold; 20 grams of nominal 9K alloy contains 7.5 grams.
net gold-bearing grams × purity decimal = fine-gold gramsThat comparison concerns metal content only. Design, maker, condition, provenance, stones and retail demand may make a lower-karat object worth more as an object than a higher-karat piece is worth for melt.
Common purity mistakes
- Treating 18K as 18% instead of 75%.
- Using 750 as 750% instead of converting it to 75% or 0.750.
- Applying a surface test to the full gross weight of a plated or mixed object.
- Counting stones and non-gold components as if they shared the marked purity.
- Assuming a hallmark proves market value, ownership, provenance or collectibility.
- Rounding too early in a multi-step calculation.