Ring Size Converter

Ring sizes are based on the inner circumference of the ring.

US/Canada sizes use a numbered scale (typically 3–16). UK/Australia uses letters (A–Z+). EU/ISO uses the inner circumference in millimeters. Japan uses a numbered scale (1–30).

Conversion formulas (using inner circumference in mm):

• US size = (circumference_mm − 36.5) / 2.55
• EU size = circumference_mm (rounded to nearest 0.25)
• Japan size = (circumference_mm − 38.0) / 0.8333
• Diameter (mm) = circumference_mm / π

Common reference sizes:

Note: Sizes are approximate. Manufacturing tolerances and different standards may cause slight variations.

The reason EU sizing feels the most logical is that it’s just the inside circumference in millimeters: a size 54 ring measures 54 mm around. US and Japan use abstract numbers and the UK uses letters, all mapping back to the same physical diameter, which is why a conversion chart is the only sane way to shop across systems. The geometry underneath is simple, circumference equals diameter times pi, so a 17.3 mm diameter works out to about 54 mm around, a US 7.

A few fitting realities the chart can’t capture. Size for your knuckle, not the base of the finger, if the knuckle is the wider part, or the ring won’t go on; just not so loose that it spins freely, which is the usual overcorrection. Band width matters too: a wide band feels tighter than a thin one at the same nominal size, so for anything over about 6 mm wide, order a quarter to half size up. And most rings can only be resized one or two sizes, while full-eternity bands and tension settings often can’t be resized at all, so getting it right the first time is worth the trouble.