💍 Ring Size Converter
Turn a finger measurement in millimetres into US, UK/AU, and EU (ISO) ring sizes — enter the inner diameter or the circumference and see them all at once.
📏 One Measurement, Every Scale
What is a Ring Size Converter?
It translates a physical finger measurement into the ring sizes used around the world. Give it the inner diameter or circumference in millimetres and it computes the US number, the UK/AU letter, and the EU/ISO size so you can order confidently from any jeweller.
Use it to match an overseas size, decode a ring you already own, or size a partner's finger for a surprise. Sizing differs slightly between makers and band widths, so for an important or expensive ring, confirm the fit with a physical sizer before ordering.
❓ Frequently Asked Questions
How does the ring size converter work?
Enter the ring's inner diameter or circumference in millimetres. The tool derives the circumference (diameter × π), then applies the standard US formula — US size = (circumference − 36.537) ÷ 2.5535 — and reports the matching EU (ISO) size and a UK/AU letter, plus the diameter and circumference.
How do I measure my ring size at home?
Two reliable ways: measure the inner diameter of a ring that already fits with a caliper or ruler, or wrap a strip of paper around the base of the finger, mark where it overlaps, and measure that length as the circumference. Enter whichever you have — the tool handles both.
What's the difference between US, UK, and EU ring sizes?
They're just different scales for the same finger. US uses numbers (with half sizes), the UK and Australia use letters (A, B, C… with half-letter steps), and the EU/ISO system uses the inner circumference in millimetres directly. This tool shows all three side by side.
How accurate is it, and should I still get sized?
It's a close guide built on standard formulas, but sizing varies slightly between jewellers and a wide band fits tighter than a thin one. For an engagement ring or any expensive purchase — especially a surprise — confirm with a physical ring sizer or an in-store fitting.