How much gold will this ring use? Estimating metal weight from a CAD file

Before you can quote a bespoke piece you need to know roughly how much metal it will take. Get it wrong upward and you lose the job to someone who quoted properly. Get it wrong downward and you fund the difference yourself.

The number is already in the model. A CAD file is a closed solid, which means the software can measure its volume exactly, and volume is one multiplication away from weight. It is worth knowing how to do that yourself, because the figure changes as the design changes, and the cheapest time to find out is before anything is cast.

The calculation

Jewellery CAD reports volume in cubic millimetres. Metal densities are quoted in grams per cubic centimetre. So the whole calculation is one division to reconcile the units, then one multiplication:

Weight in grams = volume in cubic millimetres, divided by 1000, multiplied by the density of the alloy.

The division by 1000 is the only part people get wrong, and it is worth being deliberate about, because getting it wrong moves the answer by a factor of a thousand rather than a few percent. There are 1000 cubic millimetres in a cubic centimetre.

Densities worth keeping to hand

In grams per cubic centimetre:

• Sterling silver, 925: 10.36
• 9ct gold: about 11.3
• 14ct yellow gold: about 13.1
• 18ct yellow gold: about 15.5
• 18ct rose gold: about 15.2
• 18ct white gold: anywhere from about 14.7 to 16.0, depending on whether it is a nickel or palladium white
• 22ct gold: about 17.8
• Platinum, 950: about 20.1, though platinum alloys vary more than most
• Palladium, 950: about 12.0

Treat these as close approximations rather than constants. Alloy recipes differ between refiners, and white golds differ enough that the choice of alloy is worth confirming before you quote a heavy piece. If a job is tight, ask your caster for the density of the specific alloy they pour.

A worked example

Take a plain band, 4mm wide and 1.5mm thick, in a size N with an internal diameter of 17.0mm.

• The mean diameter is the internal diameter plus the thickness, so 18.5mm.
• The mean circumference is that multiplied by pi, about 58.1mm.
• The cross-section is 4mm by 1.5mm, so 6 square millimetres.
• Volume is roughly 58.1 multiplied by 6, about 349 cubic millimetres.

Divide by 1000 and you have 0.349 cubic centimetres. That same band comes out at about 3.6g in sterling silver, 5.4g in 18ct yellow, and 7.0g in platinum. One shape, and the metal alone nearly doubles between the lightest and heaviest option.

You will not need to do this by hand for a real commission, because the software reports the volume directly. The point of working one through is to see how quickly the number moves, and which end of it you control.

The shortcut when you already have a weight in another metal

If you have cast the piece in silver as a sample, or you have the weight of the same design in a different karat, you do not need the volume at all. Divide the new density by the old one and multiply. Working from a sterling silver weight:

• To 9ct, multiply by about 1.09
• To 14ct, multiply by about 1.26
• To 18ct yellow, multiply by about 1.50
• To 22ct, multiply by about 1.72
• To platinum, multiply by about 1.94

A 6g silver sample is therefore about 9g in 18ct and about 11.6g in platinum. This is the calculation most benches actually use day to day, and it is exact in the same way the long version is, because the shape has not changed.

What the volume does not include

An accurate volume can still produce a wrong quote, because four things sit outside it.

• Stones. The volume you want is the metal only. If the stones are modelled as solids and included in the measurement, the figure will be badly high, and the error grows with the size of the centre stone.
• Sprues and gates. These are added at casting rather than modelled. The metal is recovered and reused, but your caster may quote on the sprued weight rather than the piece weight, so it is worth knowing which you have been given.
• Finishing loss. Filing, pre-polishing and polishing all remove metal. A cleanly cast piece loses a few percent; one that needs work loses more.
• Seat cutting. Setting removes metal after casting, so the piece that leaves the setter is lighter than the piece that arrived.

Cast weight and finished weight are different numbers

This is the distinction that costs money. You buy metal at the cast weight. Your customer receives the finished weight. Those are not the same figure, and the gap is not recoverable in the way sprue is, because it leaves as dust and filings.

Quote on the model weight with an allowance on top rather than on the model weight alone. How much depends on the piece and on how your finisher works, which is why the honest version of this advice is to measure your own loss across a few jobs rather than to take a number from a blog. Once you know your own figure it stops being a guess.

The levers that move the number

When a weight comes back higher than the quote allows, the design can usually absorb the difference without looking different. In rough order of how much they give back:

• Hollowing the underside of a heavy head or basket, which is invisible once set and often the single largest saving available.
• Opening the gallery, which reads as a design feature rather than a cost measure.
• Tapering the shank from the shoulders down, rather than carrying full thickness the whole way round.
• Reducing wall thickness generally, which is the smallest lever and the one with the least room in it.

There are floors on all of this. A shank much below 1.2mm at its thinnest will not survive everyday wear, and thin walls risk an incomplete pour. Your caster will have their own minimums and they are the ones to ask, because they are the ones who have to fill the mould.

Ask for the number before the model is signed off

Weight is a design decision, not a casting outcome. It is settled by the time the model is finished, and everything after that point is expensive to change.

So the useful moment to see the figure is while the file is still a file. If it comes back heavier than the job allows, adjusting a gallery or hollowing a head is a day of work and costs nothing but time. The same correction after casting costs a casting, and the metal that went into the first one is worth exactly what you paid for it minus refining.

We return the model volume and an estimated weight in your chosen alloy with every commission, so the number is in front of you while it is still free to change. Quotations are free and carry no obligation, so it costs nothing to send a design and find out what it would weigh.