EU ring size 51 in US, UK & Japan

EU/ISO ring size 51 is an inner circumference of about 51 mm (50.6 mm exactly) — that equals US 5.5, UK K½ and Japan 10, with an inner diameter of 16.10 mm.

Illustrative — screen scale varies. Use the true-scale ring below for a real measurement.

EU / ISO: 51
US / Canada: 5.5
UK / AU: K½
Japan: 10
CH / IT / ES: 11
Diameter: 16.10 mm
Circumference: 50.6 mm

Why EU 51 looks larger than US 5.5

Under ISO 8653, the EU (and ISO) ring size number is the ring's inner circumference in millimetres. So EU 51 means the band measures about 51 mm all the way around — a circumference, not a diameter. That's why EU numbers are so much larger than US numbers for the same finger, and why an EU size can be read straight off a millimetre measurement.

50 mm reference bar

True-to-scale ring Printed at 100%, the inside of this ring is 16.10 mm across.

Convert the other way with the EU to US ring size chart, open the US size 5.5 page, see the full chart, or use the converter.

Frequently asked questions

Why does EU 51 use a circumference number?

EU/ISO 51 follows the published circumference convention: about 51 mm around the inside (50.6 mm in this chart), corresponding to a 16.10 mm inner diameter.

How should I confirm EU 51 before ordering?

Check the retailer's own chart and confirm the 16.10 mm inner diameter with a physical sizer or a ring that already fits. Converted labels are a starting point, not a fit guarantee.