About SizeMyRing

SizeMyRing converts a ring's internal diameter or circumference (in millimeters) into US/Canada, UK/Ireland/Australia/New Zealand and EU/ISO ring sizes, or works the other way from a US size. It also shows two ways to measure a ring size at home without a professional sizing gauge, and a full reference table.

Where the numbers come from

Source: Wikipedia, "Ring size", "Ring dimensions in various ring size measurement systems" table, sourced to ISO 8653:2016 (fetched 2026-10-01), https://en.wikipedia.org/wiki/Ring_size. The underlying international standard, ISO 8653:2016, is a paid standard and was not directly fetchable (checked 2026-10-01). Several major jewelers' own size guides were also checked on 2026-10-01 as a possible second source: Brilliant Earth and Tiffany & Co both block automated fetches, James Allen's site now redirects entirely to Blue Nile (a brand consolidation, not a block), and Blue Nile's own guide loads but only as a printable PDF sizer with no extractable numeric table. None offered a usable second source, so this site relies on Wikipedia's freely available, ISO-8653-sourced table instead, with every row's circumference spot-checked against circumference = pi x diameter before use.

Circumference figures this calculator computes live (as opposed to the reference table's own printed values) use circumference = pi x diameter with JavaScript's Math.PI (IEEE-754 double precision, 3.141592653589793).

What this site doesn't do

It doesn't store what you enter or require an account -- every conversion runs in your browser. It reports the standards-based size for a given measurement; it doesn't adjust for band width, finger shape, or temperature-related swelling, which are judgment calls best made in person with a jeweler, especially for an expensive or non-resizable ring.

Who built this

SizeMyRing is built and maintained by Elbert Ballo. The underlying sizing standard doesn't change, but sourcing is revisited periodically -- if a major jeweler's own published chart becomes fetchable and disagrees with the table here by more than rounding, that will be noted on the page, not resolved silently.

Corrections

If a number or worked example here looks wrong, email hello@sizemyring.com. The source is named on the page so a correction can be checked against it directly.