Ring size inside the CAD file

The size is the inner diameter

A US 7 is a band 17.3 mm across on the inside; a US 9 is 18.9 mm; each full size is 0.81 mm more. Every ring-size system resolves to that diameter or to the circumference it implies. So a design's size is read off the model: measure across the inside of the band at its narrowest, in millimetres, and that is the size in any system.

Which formats carry it

STEP, IGES and Rhino 3DM carry real units. A band modelled at 17.3 mm inside is 17.3 mm in every program that opens the file, and in the studio here, which renders at that scale. An STL carries no units at all: it is a list of triangles at whatever scale the exporter assumed, with no statement of what a unit is. A ring that was 17.3 mm in Rhino can arrive as 17.3 of anything — and an STL exported in inches from a model drawn in millimetres arrives 25 times too small, which is common enough that the studio's STL page warns of it.

This is the sizing reason, on top of the stones reason, to keep the 3DM or the STEP and treat STL as the print file it is.

• STEP, IGES, BREP, 3DM, 3MF — units carried
• STL, OBJ, PLY — no units; the scale is an assumption
• glTF — metres by convention, which exporters mostly honour

Grading a design across sizes

Scaling the whole ring is wrong. A size 9 is not a size 6 enlarged by 14 per cent: the stone does not grow, the prongs do not thicken, the band's cross-section stays the same and only the loop gets longer. Graded properly, the head is fixed and the shank is re-drawn to the new inner diameter — which is why jewellery CAD tools have a dedicated resize that keeps the head, and why a shank with a pattern or side stones has to be re-laid at each size.

For rendering this matters in one specific way: the metal-range and listing packs render the piece as modelled, at one size. If a range is sold in sizes 5 to 10, the listing image is usually the middle size — a 7 for women's rings — and it is worth checking that is the size in the file before the shoot, because an eternity band at size 5 and at size 10 have visibly different stone counts.

Checking a file's size before you render

Open the file in the program that wrote it and measure the inner diameter; every CAD tool has a distance tool. If the number is 17 to 19 and the file is STEP or 3DM, it is a real ring size and the studio will render it at that scale. If the number is 0.68 or 680, the model is in inches or has been scaled, and the STL is the likely culprit. The studio frames whatever arrives, so a mis-scaled ring still renders — it just is not the size the listing claims.

Wide bands and comfort fit, in the model

A wide band needs half a size more than a slim one, and a comfort-fit band — rounded on the inside — fits nearer true. Both are decisions in the cross-section of the shank, made in CAD, and both show in a render: the Side camera shows the band's width and the Close-up its inside edge. The pose guide covers which camera shows what. Everything here happens on your own machine: the file is converted and rendered in the browser through your GPU and is never uploaded, so an unreleased design stays unreleased.

Questions