Ring Size Converter

What this does

So you have a ring size in one system, say a US number, and you need it in another, like a UK letter or a diameter in millimetres. This lines them all up. Pick your size in whichever column you know, and every other column updates to match. It covers diameter and circumference, in millimetres and inches, plus the US and Canada, UK and Australian, French and Russian, German, Japanese, and Swiss systems.

How to use it

1. Find the column for the system you already know, for example US and Canada, or Inside Diameter MM.
2. Pick your size or measurement from that dropdown.
3. Read your matching size across all the other columns.

If you have measured a ring rather than a finger, use the diameter or circumference column as your starting point.

How it works

Under the hood it is a conversion table built on standard ring sizes. When you choose a value in one column, it sets every other column to the matching row, so they always agree. The thing to understand is that a ring size is really just the inside size of the band, and every country simply labels that same physical size differently. The international standard, ISO 8653, defines the size by the inside circumference in millimetres. Circumference and diameter are tied together by a bit of school geometry, the circumference is just pi times the diameter, which is why both are offered here and why a change of even a fraction of a millimetre nudges you to the next size.

Common sizes at a glance

Here are some of the most common sizes lined up across the main systems, as a quick reference. The diameter and circumference are the inside of the band, in millimetres.

How to measure your ring size

Two easy ways at home. If you already have a ring that fits the right finger, measure straight across the inside of it, edge to edge, in millimetres, and look that up in the Inside Diameter MM column. If you do not, wrap a thin strip of paper or a piece of string around the finger, mark where it overlaps, then measure that length in millimetres, that is your inside circumference.

A few things make the measurement honest. Fingers swell through the day and in the heat, so measure when your hands are warm and towards the end of the day, not first thing on a cold morning. Make sure the size lets the ring slide over your knuckle, since the knuckle is often wider than the base of the finger. Wide bands sit tighter, so if your ring has a chunky band, lean a size up. And when you fall between two sizes, go up rather than down. For an expensive or surprise ring, it is worth getting sized properly by a jeweller.

Questions people ask

References

1. International Organization for Standardization. ISO 8653:2016, Jewellery, Ring-sizes, Definition, measurement and designation. https://www.iso.org/standard/65408.html