About WhatRingSize

WhatRingSize exists to answer one question well: what is my ring size, anywhere in the world? Most ring size tools online are either buried inside a jeweller’s shop (and nudging you to buy) or are decades-old tables that don’t work on a phone. We wanted a fast, neutral, accurate tool that just gives you the answer.

How our conversions are calculated

Every conversion on this site starts from a single physical value: the ring’s inner circumference in millimetres. From there:

• US & Canada use the ISO relationship circumference = 2.55 × US + 36.5.
• EU / ISO 8653 sizes are the circumference in millimetres.
• UK / Australia letters step by about 1.25 mm of circumference from a fixed anchor.
• Japan (JIS) sizes come from the inner diameter in ⅓ mm increments.

Because every column is derived from the same source number, the systems can’t drift out of alignment, a problem that affects a surprising number of charts online. We also cross-check against well-established jeweller reference points, such as a US 7 measuring 17.3 mm across.

Standards we follow

ISO 8653:2016 (ring sizes by inner circumference), the British/Commonwealth letter scale, and JIS S 4700:2022 (Japanese sizes). Where national charts genuinely disagree at the edges, we favour the underlying millimetre measurement, which is unambiguous.

Honest limitations

On-screen and at-home measurements are close estimates, not laboratory readings. For an engagement ring, wedding band, or any non-resizable ring, confirm the final size with a jeweller using a steel mandrel. See our terms of use for the full disclaimer.

Get in touch

Spotted an error or have a suggestion? We genuinely want to know, accuracy is the whole point. Reach us on the contact page.