How many stones fit around a ring of a given size, with the gap between them, for a full, three-quarter or half eternity band. Drawn to scale so you can see the layout before it goes to CAD.
The band built in true millimetres: ring size, stone size, count, gap and setting as calculated. Stones are ray traced; metal is polished in a jeweller's photo studio.
A section of the band unrolled, enlarged: stone, gap, stone.
Ring size to inside circumference with the same formulas as JewelLoop's ring size converter (US: inside diameter 0.458 in + 0.032 in per size; UK: A = 37.5 mm, 1.25 mm per letter; EU: the circumference itself; Japan: size 1 = 13.0 mm diameter, ⅓ mm per size).
stones = floor( inside circumference ÷ (stone width + gap) ); the leftover space is shared out, so the actual gap is circumference ÷ stones − stone width. Three-quarter and half bands take that share of the ring, rounded to an odd number so one stone sits at the centre top.
Counting on the inside circumference is the conservative trade method: the stones' girdles sit a little further out, so the finished stones end up close to girdle to girdle once set. CAD layout and the setter make the final call.
Typical gaps used by setters, by stone size:
| Stone size | Typical gap | Usual setting |
|---|---|---|
| 1.7–2.0 mm | 0.25–0.30 mm | Channel or bead |
| 2.2–2.5 mm | 0.30–0.35 mm | Shared prong or bead |
| 2.7–3.0 mm | 0.35–0.45 mm | Shared prong |
Carat weight for round brilliants uses the formula of JewelLoop's diamond tools (diameter² × depth × 0.0061, 61% depth); other shapes vary too much with depth to estimate here.
Sources: stone count and gap figures: setters' practice as discussed on the Engraver's Cafe forum ("add 0.2–0.3 mm to the diameter of the stones") and the gap table of the CarIt eternity ring calculator; resizing: GIA, Ring size guide ("very difficult to resize eternity rings"). Estimates for guidance; verify before production.