About Ring Size Calculator
A free ring sizing reference. Here is where the numbers come from and how they are checked.
Ring Size Calculator is a free reference for converting a finger measurement into a ring size, and for understanding why the answer is rarely as simple as a single number.
What this site is for
Most people arrive with one of two things. Either a measurement in millimetres and no idea what size that is, or a size in one country's system and a need to know it in another. The calculator handles both in seconds.
The guides exist because the number alone is often not enough. Measure at the wrong time of day, or ignore how wide the band is, and you get a ring that does not fit even though the arithmetic was right. Those pages cover what a converter cannot.
The guides are written and maintained by a named editor, who is accountable for the figures published here. About the editor
Where our numbers come from
Every size figure published anywhere on this site is generated from a single reference table. No page carries its own copy of the data, which means two pages cannot disagree with each other.
- US inside diameters
- The standard figures published in the widely used US jewellers' chart, running from 14.1 mm at size 3 to 22.2 mm at size 13.
- Circumference
- Calculated as diameter multiplied by pi, rounded to one decimal place. It is not transcribed from anywhere, which is why the two columns always reconcile.
- UK letters
- Matched by circumference against the published British letter scale, where A is 37.8 mm and Z is 69.1 mm. Every US size in our chart falls within 0.05 mm of the UK letter shown against it.
- EU / ISO sizes
- The inside circumference in millimetres, as ISO 8653 defines it. Where a value lands on a half millimetre we show both neighbouring sizes rather than picking one.
These relationships are checked automatically whenever the site is built. The test verifies that diameter and circumference reconcile, that each UK letter matches its own published circumference, that EU sizes round correctly, and that the data sent to the calculator in your browser is identical to the data used to print the charts.
Everything published here is listed in one place on the guide index, grouped by measuring, converting and fit.
What we do not claim
Ring sizing has no single enforced global standard. Published charts differ between manufacturers by up to about a quarter of a size. A home measurement adds error of its own on top.
So we describe our output as a close estimate, not a guaranteed fit, and we say so on every page that carries a number. For an engagement ring, a wedding band, or anything that cannot easily be resized, a jeweller with a set of metal sizing rings will beat any website, including this one.
How the site is funded
It is not, in any meaningful sense. There is no advertising, no affiliate links, no sponsored placements and no paid recommendations. Nobody pays to appear here, and no retailer is mentioned because of a commercial arrangement.
There is a Ko-fi link in the footer for anyone who wants to contribute toward hosting. That is the only monetisation on the site, and following it is entirely optional.
The conditions attached to using the site are set out in our terms and conditions.
Corrections
If you find a figure on this site that does not reconcile, or guidance you believe is wrong, we want to hear about it. Sizing data is the kind of thing that gets copied between websites without anyone checking it, and errors propagate quietly. Get in touch.