Diamond sizes in millimetres: the chart your CAD file actually needs
Carat is weight, not size. Here are the millimetre dimensions for every common cut, and why the seat should be modelled from the stone in front of you.
A CAD model does not care what a stone weighs. It needs a diameter, a length and a width, in millimetres, so a seat can be cut to fit. Carat is a unit of weight, and two stones of identical weight can differ enough in size that a seat modelled for one will not accept the other.
These tables are the working approximations most benches use. They are a starting point for quoting and modelling — not a substitute for measuring the stone you actually have.
Round brilliant
The most common cut and the most predictable, because a well-cut round has a consistent depth-to-diameter relationship.
| Carat | Diameter (mm) |
|---|---|
| 0.25ct | 4.1mm |
| 0.33ct | 4.4mm |
| 0.50ct | 5.2mm |
| 0.75ct | 5.8mm |
| 1.00ct | 6.5mm |
| 1.25ct | 6.9mm |
| 1.50ct | 7.4mm |
| 2.00ct | 8.1mm |
| 2.50ct | 8.8mm |
| 3.00ct | 9.3mm |
Melee sizes
For pavé, channel and accent work you will usually be given millimetres rather than carats, which is the sensible way round. For reference:
| Diameter (mm) | Approx. carat |
|---|---|
| 1.0mm | 0.005ct |
| 1.3mm | 0.01ct |
| 1.5mm | 0.015ct |
| 1.7mm | 0.02ct |
| 2.0mm | 0.03ct |
| 2.4mm | 0.05ct |
| 3.0mm | 0.10ct |
Melee is normally supplied sieved to a size range rather than cut to an exact figure, so a parcel sold as 1.3mm will contain stones a little either side. This matters for a pavé field, where hole spacing was modelled to one number.
Fancy shapes at one carat
Fancy cuts vary far more than rounds, because length-to-width ratio is a matter of taste rather than optics. These are typical dimensions for a 1.00ct stone:
| Cut | Length × width (mm) |
|---|---|
| Oval | 7.7 × 5.7mm |
| Pear | 8.5 × 5.5mm |
| Marquise | 10.5 × 5.0mm |
| Emerald | 7.0 × 5.0mm |
| Princess | 5.5 × 5.5mm |
| Cushion | 5.8 × 5.8mm |
| Radiant | 6.5 × 5.5mm |
| Asscher | 5.5 × 5.5mm |
| Heart | 6.5 × 6.5mm |
Note how differently a carat presents. A marquise at 10.5mm long looks far larger than a princess at 5.5mm square, and they weigh the same. Clients comparing stones on weight alone are usually surprised by this.
Baguettes and tapered baguettes
Step cuts for side stones are sold by millimetre dimensions rather than carat, because their proportions are dictated by where they have to sit. A pair flanking a centre stone will be specified as, say, 3.0 × 1.5mm, and tapered baguettes will carry both widths.
Always model these from the supplied dimensions. There is no useful carat conversion.
Why the same carat gives different millimetres
Three things move the size of a stone at a fixed weight.
- Depth. A deep-cut stone hides weight below the girdle and looks smaller from above. A shallow one spreads wider for the same carat.
- Girdle thickness. An extremely thick girdle adds weight that contributes nothing to the visible size.
- Length-to-width ratio. On any fancy shape this is a preference, not a standard, so two 1ct ovals can differ noticeably.
This is precisely why a chart cannot be the last word. It gets you to a quote; the stone gets you to a seat.
What the CAD file needs
For a seat that fits first time, send dimensions rather than weights.
- Round: the diameter in millimetres.
- Fancy shapes: length and width, both in millimetres.
- Depth, where you have it — it decides how deep the seat and how tall the gallery must be.
- Girdle thickness if the stone is unusual, because a very thick girdle changes where the prongs must grip.
- For melee: the sieve size, and whether the parcel is calibrated.
If the stone is already in hand, measure it. If it is still being sourced, model to a confirmed target size and treat any substitution as a file revision rather than a detail.
The seat is cut for one stone
A seat modelled for a 6.5mm round will not take a 6.6mm one. That is a tenth of a millimetre, well inside the range a chart glosses over, and it is the difference between a setter working and a setter calling you.
So the useful discipline is simple: quote from the chart, model from the stone.
We model seats to the actual dimensions you supply and confirm them back before anything is printed. Quotations are free and carry no obligation, so send the design and the stone details and we will check the fit with you first.
