Ring sizing for CAD: US, UK and Indian sizes, and the millimetres that matter
Size letters and numbers mean nothing to a model. Here is how they convert, and why the internal diameter is the only figure to send.
Ring size is the single most common cause of a remake, and almost always for the same reason: a size was sent in one country's system and read in another's. A CAD model does not know what size N means. It knows diameters in millimetres, and that is the number worth agreeing on.
Three systems, one measurement
Every sizing system is describing the same physical thing — the internal circumference or diameter of the band — in a different notation.
- UK and Australia use letters, A through Z with halves.
- The US and Canada use numbers, roughly 3 to 13 with halves and quarters.
- India, China and Japan use numbers on a different scale entirely, so an Indian 12 is not a US 12.
- Europe generally uses the internal circumference in millimetres directly, which is the least ambiguous of all.
A working conversion table
Internal diameter in millimetres, with the common equivalents:
| Diameter (mm) | UK | US | India |
|---|---|---|---|
| 14.1mm | G | 3.5 | 5 |
| 15.3mm | J½ | 5 | 9 |
| 16.1mm | L½ | 6 | 12 |
| 16.9mm | N½ | 6.75 | 14 |
| 17.3mm | O½ | 7.5 | 15 |
| 18.1mm | Q½ | 8.5 | 17 |
| 18.9mm | S½ | 9.5 | 19 |
| 19.8mm | U½ | 10.5 | 22 |
Treat these as close rather than exact. Conversion charts differ slightly between suppliers, which is precisely why the millimetre figure should travel with the order rather than the letter.
Circumference or diameter — say which
A ring quoted at 54 is almost certainly 54mm of internal circumference, which is about 17.2mm in diameter. A ring quoted at 17 is almost certainly the diameter. Sending the number without the unit is how a size lands three sizes out.
Circumference divided by pi gives diameter. It is a small piece of arithmetic that prevents a large mistake.
Band width changes the size that fits
A wide band feels tighter than a narrow one at the same measured size. The usual working rule is to go up about a quarter size for bands over 6mm wide, and up to a half size for anything over 8mm.
This matters because the client was probably measured on a narrow sizer. If the finished ring is a wide court band, the size they were measured at is not the size they need.
Comfort fit is a different measurement again
A comfort-fit band has a domed inner surface, so it slides over the knuckle more easily and fits looser at the same nominal size. Most makers size comfort fit a quarter to a half size down from standard fit.
If the brief says comfort fit, confirm whether the size given is already adjusted or not.
Sizing and shrinkage compound
Casting shrinkage pulls the finished ring smaller than the model. A piece going through a rubber mould loses more than one printed and cast directly. Combine a size sent in the wrong system with unallowed-for shrinkage and the result can be a full size out.
What to send with the order
- The internal diameter in millimetres, stated as diameter.
- The system the client was measured in, as a cross-check.
- The finished band width.
- Whether the fit is standard or comfort.
- Whether the piece will be printed and cast directly, or moulded.
Five lines, and they remove almost every sizing remake.
Send millimetres, not letters
Letters and numbers are conveniences for the counter. The model needs a diameter, and the caster needs to know what will happen to it. Send both and the ring comes back the size you asked for.
We model to a stated internal diameter and confirm it back before anything is printed. Quotations are free, so send the design and the size and we will check it with you first.
