Casting Weight Calculator
Metal needed from a wax model's weight.
Calculate the metal weight a wax model will cast into, by density ratio, including sprue and button allowance.
Metal needed
96.88 g
77.50 g in the piece plus sprue
The sprue and button are recovered and reused, but they have to be in the flask, so they are bought up front. Expect one to two percent loss per melt from oxidation, more with repeatedly reused scrap.
How the Casting Weight Calculator works
Lost wax casting has one convenient property: the metal weight is the wax weight multiplied by the ratio of their densities, because the metal occupies exactly the space the wax did. Wax is close enough to 1 g/cm³ that the ratio is simply the metal's density — 15.5 for 18k gold, 10.4 for sterling.
Also known as: wax to metal calculator · lost wax weight calculator
Why the ratio is just the density
In lost wax casting the metal occupies precisely the space the wax did, so the metal weight is the wax weight times the ratio of their densities. Casting wax is close enough to 1 g/cm³ that the ratio is simply the metal's density: 15.5 for 18k gold, 10.36 for sterling, 20.1 for platinum.
That makes the estimate unusually reliable for a workshop calculation. The uncertainty is in the wax, not the metal — injection wax runs slightly under 1 and carving wax slightly over, which shifts the answer by a percent or two.
The sprue and button are added on top as a percentage. They are recovered and reused, but they have to be present in the flask, so they have to be bought or supplied before the pour.
A worked example
A five gram wax model in 18k gold: five times 15.5 is 77.5 grams of metal in the piece. Adding 25% for sprue and button gives 96.9 grams to melt, which is 3.1 troy ounces or 62 pennyweights.
The same model in sterling is 51.8 grams plus sprue, and in platinum 100.5 grams plus sprue. The metal choice changes the material cost by more than an order of magnitude, and the wax weight is the only thing that stayed the same.
Where the answer misleads
Melt loss is real and cumulative. Expect one to two percent per melt from oxidation and handling, and more from repeatedly reused scrap that has picked up contamination. Casters usually build this into a quote rather than itemising it.
The density figures are for nominal alloys, and alloys vary. An 18k white gold with palladium is denser than one with nickel, and both are sold as 18k. For a one-off the difference is noise; across a production run it is worth measuring.
Finally, this estimates metal weight, not cost. Casting fees, finishing, stone setting and loss all sit on top, and for a small piece the labour usually exceeds the metal by a wide margin.
Frequently asked questions
How do I calculate casting weight from wax?
Multiply the wax weight by the metal's density. Five grams of wax casts into 77.5 g of 18k gold at a density of 15.5, or 51.8 g of sterling silver at 10.36.
How much extra for the sprue?
Twenty to thirty percent is typical for the sprue and button, depending on the piece and the caster's setup. It is recovered and reused, but it has to be in the flask, so it has to be bought or supplied up front.
Do I need to allow for loss?
A small amount, usually one to two percent per melt from oxidation and handling, and more with repeated reuse of scrap. Casters typically factor this into their quote rather than charging it separately.
Does the wax type matter?
Slightly. Injection wax runs a little under 1 g/cm³ and carving wax a little over, so the ratio shifts by a percent or two. For a costing estimate that is noise; for a large production run it is worth measuring your actual wax.