A setting is quoted for a range of centre stones. Go outside it and the stone does not sit: the girdle rides up on the prongs, or it drops through. This is the question almost no size guide answers.
Retailers describe a head by the carat range it accepts, so that is how these are held here. The millimetres are derived with the same weight-to-dimension conversion the rest of the site uses, which is why the seat figures below and the tool always agree.
The seat is a range because the head is made in a range. When you pick a style in the tool it works out the one seat your stone should be ordered in, which is a figure inside the span below rather than any fixed point in it.
| Style | Quoted for | Seat range | What it means |
|---|---|---|---|
| Solitaire | 0.75–2.00 ct | 5.85–8.11 mm | Classic single stone. Shows the most of the girdle. |
| Halo | 0.50–1.50 ct | 5.11–7.37 mm | Accent stones ring the centre. Only the centre seat is modelled. |
| Three stone | 0.75–2.00 ct | 5.85–8.11 mm | Side stones flank the centre, which tightens the seat lengthwise. |
| Bezel | 0.50–1.50 ct | 5.11–7.37 mm | A metal rim fully surrounds the girdle. Least tolerance for a misfit. |
| Cathedral | 0.75–2.00 ct | 5.85–8.11 mm | Shoulders sweep up to the head, so the stone sits higher. |
| East–west | 1.00–2.50 ct | 7.91–10.74 mm | An elongated stone turned across the finger rather than along it. |
These are approximate reference values, not manufacturer specifications. Typical clearances run a tenth to half a millimetre, so half a millimetre of error flips the answer. Use the figure from the actual product page whenever you have one — the tool takes a seat diameter directly.
A round seat is limited by the stone's width in every direction. A three-stone head is tightened lengthwise by its side stones, and an east-west setting turns an elongated stone across the finger rather than along it. So the dimension that has to fit changes with the style, not only with the size: an oval that sits happily in one may foul another at exactly the same carat.
Check a stone against a seat →