Ring Size Conversion: US, UK, EU and the Millimetres That Matter
Only one ring-size number is standardised: the inner circumference in millimetres. US numbers, UK letters and EU sizes are all conventions layered on top of it.
1 November 2026 · 6 min read · Reference
A ring size is the one measurement people routinely guess, and guessing is why so many rings come back for resizing. The fix is to stop thinking in sizes and start thinking in millimetres, because underneath every national scale sits a single number: the distance around the inside of the band.
One number, many names
The international standard, ISO 8653:2016, defines a ring size as the inner circumference of the band in millimetres. A ring that measures 54 mm around the inside is an ISO size 54, full stop. Every other system is a convention layered on that one measurement:
- The United States and Canada use numbers, roughly 3 to 13 with half sizes. The scale is defined by a formula: the inner circumference in millimetres equals 36.537 + 2.5535 × size. So each full US size adds about 2.55 mm of circumference, or 0.81 mm of diameter — almost exactly one thirty-second of an inch.
- Continental Europe mostly quotes the ISO circumference directly, so an "EU 54" is simply a 54 mm ring.
- France, Spain, Italy and Switzerland quote the circumference minus 40. An ISO 54 ring is a French 14; an ISO 50 is an Italian 10.
- The United Kingdom, Ireland and Australia use letters, A through Z with half sizes, again stepping along the same circumference.
Because they all measure the same thing, you can always move between them through millimetres. That is also why a converter that works in circumference — the ring size converter — can hold every scale at once.
The reference chart
These are the figures the US formula produces, rounded the way jewellers round them. Diameter is the circumference divided by π.
| US / Canada | Diameter (mm) | Inner circumference (mm) · ISO / EU | France · Spain · Italy |
|---|---|---|---|
| 4 | 14.9 | 46.8 | 7 |
| 5 | 15.7 | 49.3 | 9 |
| 6 | 16.5 | 51.9 | 12 |
| 7 | 17.3 | 54.4 | 14 |
| 8 | 18.1 | 57.0 | 17 |
| 9 | 18.9 | 59.5 | 20 |
| 10 | 19.8 | 62.1 | 22 |
| 11 | 20.6 | 64.6 | 25 |
| 12 | 21.4 | 67.2 | 27 |
| 13 | 22.2 | 69.7 | 30 |
A worked example: a US 7 is 54.4 mm around the inside and 17.3 mm across. On a European chart that is a 54; in France it is a 14. Convert either direction with the US size to circumference converter.
The UK letter scale
The UK scale is the one that trips people up, because letters carry no arithmetic you can do in your head. It still tracks circumference: the jeweller Goldsmiths lists size A at 37.8 mm (11.9 mm across), and each successive letter adds about 1.25 mm of circumference, running up to Z at roughly 68.5 mm. Half sizes — A½, B½ and so on — fall between.
The catch is the US equivalent. Major jewellers do not agree on it to better than about half a size. For a US 6, the H. Samuel chart lists UK M, while the Goldsmiths scale works out closer to L½; for a US 7, H. Samuel gives O where the circumference points to N½. Neither is wrong — the letter scale and the number scale do not line up cleanly, so each retailer rounds its own way.
The practical rule follows from that: if a chart gives you a UK letter and a US number that disagree, trust the millimetres. Ask the seller for the inner circumference or diameter in millimetres and convert from that, not from the letter.
How to measure without a jeweller
The reliable method measures a ring that already fits, not your finger:
- Take a ring the right finger already wears comfortably.
- Measure its inner diameter across the middle in millimetres, with a ruler or calipers.
- Convert with the diameter to US size converter, or multiply the diameter by π (about 3.14) to get the circumference.
If you must measure the finger directly, wrap a strip of paper or a length of string around the base of the finger, mark where it overlaps, and measure that length in millimetres — that is the circumference, ready to read straight off the ISO scale. Our printable ring sizer gives you a scale cut to the right size to do exactly this.
Two things to get right:
- Measure when the finger is warm, at the end of the day. Fingers shrink in the cold, and a ring sized to a cold finger will be tight in summer.
- Size up for a wide band. A band more than about 6 mm wide sits tighter, so add a quarter to half a size. The knuckle matters too: if it is much larger than the base of the finger, size to clear the knuckle.
Why rings still come back
Converting correctly gets you to the right size; it does not guarantee a perfect fit, for reasons no chart can carry:
- The same finger changes. Heat, salt, exercise and the time of day can move a finger by up to half a size across a single day.
- Resizing has limits. Most plain bands can be moved two sizes either way, but rings set with stones all the way round — eternity rings — often cannot be resized at all, which makes getting the size right first time matter more.
- Charts round differently. A gap of a few tenths of a millimetre between two retailers' tables is normal; it is the rounding, not an error.
When in doubt, a jeweller's steel ring stick measured against a ring that already fits is still the most accurate check there is.
Related
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy