ISO 8653 (metric)
JewelryWiki Editorial·2 sources·Reviewed 2026-08-04
The size number is the inner circumference in millimetres — a size 51 ring measures 51 mm around the inside. The only system on this page that defines its own instrument, its tolerances, and where on the ring to take the reading.
Properties
| Property | Value | Authority |
|---|---|---|
| How a size is written | A number from 41 to 76. ISO defines ring-size as “circumference of the bigger cylinder that could be inserted in the ring” — note that this is the largest inscribed cylinder, not simply the inner circumference, which is what makes the definition work for rings that are not truly round. Table 1 gives the corresponding ring-stick diameters: size 41 is 13,05 mm, 51 is 16,23 mm, 61 is 19,42 mm, 71 is 22,60 mm and 76 is 24,19 mm. Multiplying any of those by π returns the size number to within 0,01 mm, so the designation is the inner circumference in millimetres. [1] | — |
| Standard status | Current international standard, second edition, published 15 January 2016. It cancels and replaces ISO 8653:1986, which it technically revised. BSI records its status as “Current, Under Review”. Prepared by ISO/TC 174, Jewellery. [2] | — |
| Defining body | ISO, through technical committee TC 174 (Jewellery). [1] | — |
| Where the ring is measured | Defined explicitly, and differently for two profiles. On a ring with a ROUNDED inside profile the contact and measurement point is in the middle of the ring’s thickness; on a FLAT inside profile it is at the bottom of the thickness. The standard also specifies positions for particular shapes: a signet ring or ring with a mounting is laid with its flat part on the flat surface of the ring stick; a ring of several interlaced circles is read under the bottom; a square ring is placed so that the circle is inscribed in the square, with the flat sides of the ring NOT on the flat sides of the stick. [1] | — |
| Stated tolerance | Yes. The metric ring stick must have a measurement scale 160,0 mm ± 0,5 mm long, must be metal or a material of equivalent wear resistance, and each size diameter carries a tolerance of ±0,02 mm. The ring is slid down the stick “without any external force”, and the stick itself must be checked regularly against a finger gauge set. [1] | — |
| What to watch for | The standard governs measuring RINGS, largely during manufacture. Measuring a finger is treated as a separate matter: ISO notes only that for jeweller-to-consumer work the finger is sized with a gauge set whose rings share the diameters and tolerances of the ring stick. So a size measured on a finger and a size measured on a ring are made comparable by the tolerance, not by the same procedure. [1] | — |
Sources
- ISO 8653:2016 — Jewellery — Ring-sizes — Definition, measurement and designation — International Organization for Standardization (ISO 8653:2016(E), Clauses 2, 3 and 4; Table 1)
- BSI Knowledge — catalogue entries for ISO 8653:2016 and BS EN 28653:1993 — British Standards Institution (BSI Knowledge, standard status records)