Approximate EU and US ring sizes from inside diameter.
| EU | – |
| US | – |
Use this when you have a well-fitting ring and a caliper, or a printed sizer. EU size is the inner circumference in millimetres: EU = π × inside diameter. US size uses (diameter − 11.63) / 0.812. 17.3 mm inside is about EU 54.4 and US 7. Brands last differently. Measure the inside of a ring, not the finger circumference unless you divide by π first. Diameter can be millimetres or inches.
Inside Ø: Inside diameter of the ring.
EU: ISO / European size (inner circumference in mm).
US: Approximate US numerical size.
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