Gold Alloy Karat Calculator
Find how much alloy to add to reduce a piece of gold to a lower karat, keeping the pure-gold content fixed — the standard karating-down calculation.
How it works
Pure gold = weight × current karat ÷ 24 · New total = pure gold × 24 ÷ target karat · Alloy to add = new total − weight
Karat measures the parts-per-24 of pure gold, so lowering karat means diluting the same gold with more base metal. The pure-gold weight never changes; you simply add alloy until it becomes a smaller fraction of the total. Because pure gold is worth far more than alloy, karating down stretches your gold further, which is exactly why lower-karat alloys are cheaper. You can only reduce karat this way — raising it requires adding pure gold, not alloy.
Worked example
10 g of 24K gold contains 10 g of pure gold. To reach 18K the total must become 10 × 24/18 = 13.33 g, so you add 13.33 − 10 = 3.33 g of alloy.
Frequently asked questions
Can I raise karat by adding something?
Not with alloy. Raising karat requires adding pure (24K) gold to increase the gold fraction. Adding base metal only lowers karat.
What is the alloy made of?
It depends on the colour you want: copper and silver for yellow gold, more copper for rose gold, nickel or palladium for white gold. The alloy is what gives coloured golds their hue and hardness.
Does this account for melt loss?
No. Real melting and casting lose a little metal to oxidation and skimming, so weigh slightly extra alloy and verify with an assay for critical work.
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This calculator is for educational purposes. Double-check important results before acting on them.