Ask about ring sizing on Reddit and you get the same answer every time: wrap a string around your finger and measure it. That answer is why so many rings get returned. String stretches. It also sits loose where a metal band sits tight, and the difference runs to about half a size. If you only take one thing from this page, take that.
What a ring size actually is
A ring size is the inside circumference of the band, measured in millimetres. That is the whole system (ISO 8653 if you want to look it up). US size 8 means an inside circumference of 57.0 mm. Everything else, letters, US numbers, EU numbers, is a label pasted on top of that measurement.
The method that works at home
Take a ring that already fits the finger you are buying for. Lay it on a ruler and measure the widest inside distance in millimetres. That is the inside diameter. Multiply by 3.14 for the circumference, then read the table below. A ring you already wear beats any string measurement because it has already survived the argument with your knuckle.
No ring to measure? Use a strip of paper instead of string. Paper does not stretch. Wrap it at the base of the finger, mark the overlap, measure. Then check the same wrap over the knuckle. If the knuckle number is much larger, size to the knuckle, not the base. The ring has to get past it.
US to EU conversion, the short table
| US | Inside circumference | EU |
|---|---|---|
| 6 | 51.9 mm | 52 |
| 7 | 54.4 mm | 54–55 |
| 8 | 57.0 mm | 57 |
| 9 | 59.5 mm | 59–60 |
| 10 | 62.1 mm | 62 |
Three things the threads skip
Fingers change size through the day. Measure in the evening, warm, because that is your finger at its largest. Cold morning fingers run a half size smaller.
Wide bands wear tighter than thin bands at the same size. If the band is 8 mm or wider, consider going up half a size.
Open and adjustable bands forgive mistakes. Our Thorn Signet and Spine Band both adjust. Fixed bands like the Retro Thorn Ring need the real number, sizes 6 through 10.
Buying two rings at once? The Ring Stack pairs a sized band with an adjustable one, so only one number has to be right.