How to use the gold alloy calculator
- Choose your task. Target batch works backwards from a finished weight. Adjust purity starts with metal you own. Mix pieces calculates the combined purity of known inputs.
- Set the purity format and weight unit. Enter karats on a 24-part scale, or measured fineness in parts per thousand. Changing the unit converts entered weights.
- Enter the relevant weights and purities. A source with no gold has a purity of zero. Use the actual assay for fine gold when known, for example 999.9 rather than an ideal 1000.
- Calculate and review the breakdown. Check the source quantities, contained fine gold and final purity. Copy the result to keep your batch notes.
For background on the numbers, read our comparison of 24K, 22K and 18K gold. All three modes use mass fractions, so the calculation works independently of currency or a live gold-price feed.
Gold alloy mixing formulas
Let W be weight and F the fraction of fine gold. Convert karat to a fraction with K ÷ 24; convert fineness with fineness ÷ 1000. For example, 18K and 750 both give F = 0.75.
1. Build a target batch from two sources
Source B weight = W − Source A weight
The target must lie between the source purities. Use a high-purity gold source and a gold-free master alloy, or two known gold alloys. Equal-purity sources do not determine a unique mixing ratio.
2. Raise or lower the purity of existing gold
Final weight = W + Metal to add
To lower purity, the added metal must be below the target. To raise purity, it must be above the target. Adding metal at exactly the target purity cannot bring a different starting purity to that target in a finite amount.
3. Find the purity of mixed pieces
Combined gold fraction = Total fine gold ÷ Total weight
Combined karat = Combined gold fraction × 24
A heavier piece contributes more to the result. This is why two different karats give their simple average only when their weights are equal.
Three common gold mixing calculations
Make 100 g of 18K
75 g + 25 gCombine ideal 24K gold with zero-gold alloy. The finished batch contains 75 g fine gold.
Lower 10 g of 22K
Add 2.2222 gAdd zero-gold alloy to reach 18K. Final theoretical weight: 12.2222 g.
Mix 18K and 14K
16K combined10 g of nominal 18K plus 10 g of nominal 14K gives 20 g at 16K, containing 13.3333 g fine gold.
Measured-fineness example: to make a 100 g batch at 750 from 999.9 gold and a zero-gold alloy, use approximately 75.0075 g of the gold source and 24.9925 g of alloy. The small difference from 75 g accounts for the source being 99.99% gold.
Karat vs fineness: keep the input precise
Nominal karats and hallmark fineness values are not always mathematically identical. Enter 14 in karat mode to use 14/24, or 585 in fineness mode to use 58.5%. The calculator keeps these distinct.
| Nominal karat | K ÷ 24 as % | Fineness reference |
|---|---|---|
| 24K | 100% ideal | 999 or 999.9 stock: enter actual assay |
| 22K | 91.6667% | 916 = 91.6% |
| 18K | 75% | 750 = 75% |
| 14K | 58.3333% | 585 = 58.5% |
| 10K | 41.6667% | 417 = 41.7% |
| 9K | 37.5% | 375 = 37.5% |
These are calculation references, not a list of legally permitted marks in every country. A calculated result does not certify a hallmark or confirm an item’s composition.
What the result includes
The result assumes accurate input weights, known gold fractions, complete mixing and no metal loss. It reports contained gold and theoretical starting quantities. It excludes stones, non-metal parts, refining fees, price premiums, tax and processing costs.
Actual melting may change both mass and composition. A single percentage loss does not establish the final purity if different metals are lost at different rates. For this reason, the calculator does not add a universal melting-loss adjustment or promise a finished alloy colour.
Gold-free master alloy means zero gold, not one specific recipe. Its silver, copper or other metal proportions affect the finished material. Confirm suitable alloy recipes with the supplier and verify the finished batch by appropriate assay. Our guide to how gold refining changes purity explains the distinction between alloying and removing impurities.
Weight conversions use 1 troy ounce = 31.1034768 g and 1 tola = 11.6638038 g. A troy ounce is different from an everyday avoirdupois ounce.
Gold alloy calculator FAQs
How much alloy do I add to 24K gold to make 18K?
For an ideal 100% gold input and zero-gold alloy, add one-third of the starting gold weight. For 10 g of ideal 24K, that is 3.3333 g of alloy. Enter actual fineness if your gold is 999 or 999.9.
Can I mix 22K and 14K gold to make 18K?
Yes, on the nominal karat scale equal weights give 18K. For a 100 g batch, use 50 g of each. Enter measured fineness instead when the actual alloys differ from their nominal karats.
Can adding alloy turn 18K into 22K?
A gold-free alloy lowers purity. To raise 18K to 22K by adding metal, the added source must be purer than 22K. Use Adjust purity to calculate the amount.
Is 585 gold exactly 14K?
585 means 58.5% gold, equivalent mathematically to 14.04K. Nominal 14K is 58.3333%. Use fineness mode when 585 is the value you intend to calculate.
Does mixing gold remove impurities?
No. Mixing changes the proportions of the materials present. Refining removes unwanted materials through a separate process. Adding gold can increase the gold fraction without removing the original non-gold metals.
Can this calculator tell whether my gold is real?
No. It calculates from the purity you provide. Plated, filled, soldered or repaired items can have a different overall composition from a surface mark. Obtain suitable testing before relying on a batch calculation.
Continue learning about gold
Method and references
The tool uses conservation of fine-gold mass. For industry context and hallmarking information, consult:
- Morris & Watson: fabrication and alloying calculators
- British Hallmarking Council: hallmarking guidance