Purity Fraction
The purity fraction is the decimal share of pure gold in an alloy, such as 0.916 for 22K, used in price calculations.
The purity fraction is the karat value written as a decimal so it can be used in arithmetic. 22K becomes 0.916, 18K becomes 0.75, and 14K becomes about 0.585. It is the multiplier that strips the alloy out of a weight, leaving just the pure gold that the market actually prices.
Turning karat into a number you can multiply
Karat is a ratio out of 24. Dividing by 24 turns it into a fraction: 22 divided by 24 is 0.9166, rounded to 0.916, and 18 divided by 24 is 0.75. These are the same numbers stamped as fineness in hallmarks, 916 and 750, just read as a decimal.
The fraction is the bridge between a description and a price. Gold rates are quoted for pure metal, but a piece is rarely pure. Multiplying a weight by its purity fraction converts the physical weight into the equivalent pure gold content that the rate applies to.
Its role in the metal value formula
Metal value is net weight times purity fraction times the pure gold rate. The fraction is the middle term, and getting it right is what stops a 22K piece being priced as if it were solid 24K.
Take a 12 gram 18K ring with the 24K rate at 6,300 rupees per gram. The pure gold content is 12 times 0.75, which is 9 grams, worth 56,700 rupees. Skip the fraction and you would charge for 12 grams of pure gold, overpricing the ring by a quarter. That is why the purity fraction is a fixed, non negotiable input in any live pricing setup.
- The purity fraction is the karat value expressed as a decimal.
- 22K is 0.916, 18K is 0.75 and 14K is about 0.585.
- It converts physical weight into equivalent pure gold content.
- Omitting it prices lower karat gold as if it were pure.
Purity Fraction, in short.
How do I work out the purity fraction for any karat?
Divide the karat by 24. So 22K is 22 divided by 24, which is 0.916, and 18K is 18 divided by 24, which is 0.75. The result is the decimal share of pure gold.
Is the purity fraction the same as fineness?
They express the same purity. Fineness states it in parts per thousand, such as 916, while the purity fraction states it as a decimal, 0.916. One is just the other divided by a thousand.
Why not just use a 22K rate directly?
You can, and some jewellers do. But keeping the pure gold rate and applying the purity fraction is cleaner when a catalogue mixes 22K, 18K and 14K, since one rate then serves every purity.
Related terms
See purity fraction inside a real jewellery system.
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