ISO Ring Size Standards Explained
There is one international standard for ring sizing, and it is refreshingly simple: measure the inside of the ring in millimetres.
Quick answer
ISO 8653 defines the ring size as the inside circumference in millimetres. A ring measuring 54 mm inside is ISO/EU 54 — a US 7 and a UK N½.
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Inside circumference, mm
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Matched inside diameter 17.30 mm · circumference 54.35 mm
Conversion table
| US | EU (ISO) | Inside diameter | Circumference |
|---|---|---|---|
| 3 | 44 | 14.00 mm | 44.0 mm |
| 3.5 | 45 | 14.40 mm | 45.2 mm |
| 4 | 47 | 14.80 mm | 46.5 mm |
| 4.5 | 48 | 15.20 mm | 47.8 mm |
| 5 | 49 | 15.60 mm | 49.0 mm |
| 5.5 | 50 | 16.00 mm | 50.3 mm |
| 6 | 52 | 16.45 mm | 51.7 mm |
| 6.5 | 53 | 16.90 mm | 53.1 mm |
| 7 | 54 | 17.30 mm | 54.3 mm |
| 7.5 | 56 | 17.70 mm | 55.6 mm |
| 8 | 57 | 18.10 mm | 56.9 mm |
| 8.5 | 58 | 18.50 mm | 58.1 mm |
| 9 | 59 | 19.00 mm | 59.7 mm |
| 9.5 | 61 | 19.40 mm | 60.9 mm |
| 10 | 62 | 19.80 mm | 62.2 mm |
| 10.5 | 63 | 20.20 mm | 63.5 mm |
| 11 | 65 | 20.60 mm | 64.7 mm |
| 11.5 | 66 | 21.00 mm | 66.0 mm |
| 12 | 67 | 21.40 mm | 67.2 mm |
| 12.5 | 68 | 21.80 mm | 68.5 mm |
| 13 | 70 | 22.20 mm | 69.7 mm |
What the standard says
ISO 8653 sets out terminology and sizing for finger rings. The designated size is the inside circumference in millimetres, measured on the smallest inner section of the band.
Because the definition is a physical measurement rather than an arbitrary index, ISO sizes convert cleanly to any other system: divide by π for the diameter, then look up the country scale.
Who uses it
- France, Germany, Italy, Spain, the Netherlands and Scandinavia use ISO numbers directly.
- The UK, US, Japan, Switzerland and Brazil use national scales, all derived from the same physical measurement.
- Most international online retailers list ISO/EU sizes alongside their local scale.
Reading a hallmark or stamp
A ring stamped '54' inside the band is an ISO size — 54 mm inside circumference. A ring stamped 'N½' is UK, and a ring stamped '7' with no other marks is most likely US. If in doubt, measure the inside diameter and convert.
Frequently asked questions
Is ISO ring size the same as EU ring size?
In practice, yes. The European numeric size is the ISO inside circumference in millimetres.
How do I convert ISO to US?
Divide the ISO number by 3.14159 to get the diameter in mm, then look that diameter up in the conversion chart. ISO 54 is 17.2 mm, a US 7.
Keep reading
For an expensive ring, confirm the size with a jeweller before ordering — see the ring size FAQs for resizing advice.