Millimeters are the most reliable way to describe ring size because every sizing system, whether US numbers, UK letters, or EU circumference values, is ultimately derived from the inner diameter or circumference in mm. If you know your size in mm, you can order a ring from any country without relying on a conversion calculation.
This page gives you the complete reference: inner diameter and inner circumference in mm for every US, UK, and EU size.
Complete Ring Size mm Chart
| US size | UK size | EU size | Inner diameter (mm) | Inner circumference (mm) |
|---|---|---|---|---|
| 3 | F | 44 | 14.1 | 44.2 |
| 3.5 | G | 45.5 | 14.5 | 45.5 |
| 4 | H | 46.8 | 14.9 | 46.8 |
| 4.5 | I½ | 48 | 15.3 | 48.0 |
| 5 | J½ | 49.3 | 15.7 | 49.3 |
| 5.5 | K½ | 50.6 | 16.1 | 50.6 |
| 6 | L½ | 51.9 | 16.5 | 51.9 |
| 6.5 | M½ | 53.1 | 16.9 | 53.1 |
| 7 | N½ | 54.4 | 17.3 | 54.4 |
| 7.5 | O½ | 55.7 | 17.7 | 55.7 |
| 8 | P½ | 57.2 | 18.2 | 57.2 |
| 8.5 | Q½ | 58.5 | 18.6 | 58.5 |
| 9 | R½ | 59.7 | 19.0 | 59.7 |
| 9.5 | S½ | 61 | 19.4 | 61.0 |
| 10 | T½ | 62.2 | 19.8 | 62.2 |
| 10.5 | U½ | 63.5 | 20.2 | 63.5 |
| 11 | V½ | 64.8 | 20.6 | 64.8 |
| 11.5 | W½ | 66 | 21.0 | 66.0 |
| 12 | X½ | 67.2 | 21.4 | 67.2 |
| 13 | Z | 69.7 | 22.2 | 69.7 |
How to Measure Your Finger in mm
To use this chart, you need your inner circumference or inner diameter in millimeters. The easiest home method: cut a thin strip of paper, wrap it snugly around the base of your finger, mark where it overlaps, then measure that length in mm with a ruler. That measurement is your circumference and also your EU size.
Then find your circumference in the last column of the chart above and read across to find your US, UK, and EU size. For a visual guide with photos, see our full guide on how to measure ring size with string or paper.
How to Measure an Existing Ring in mm
If you have a ring that already fits the correct finger, place it flat on a ruler and measure from one inner edge to the opposite inner edge. That is your inner diameter in mm. Find it in the fourth column of the chart and read across.
Alternatively, use our screen-based ring size calculator to compare the ring against a calibrated circle on your phone. This is usually faster than a ruler for ring comparisons and is accurate enough for most purchases.
Why mm Is the Most Reliable Sizing Unit
US sizes are an arbitrary scale. UK letters are a named scale. Both can cause confusion when a brand uses a slightly different rounding convention. The inner diameter in mm is a physical measurement that does not depend on any country's convention. A jeweler in Japan, India, or Germany can all work from your mm diameter without needing to know your US or UK size.
This is why it is worth noting your mm measurement whenever you size yourself, even if you only plan to shop domestically. It is a number that stays useful across your lifetime of ring purchases. For context on why international sizing systems diverged historically, see why ring sizes differ between countries.
FAQ
What is the inner diameter of a US size 7 ring in mm?
A US size 7 ring has an inner diameter of approximately 17.3mm and an inner circumference of about 54.4mm.
How do I find my ring size from a mm measurement?
Measure your inner circumference in mm (wrap a paper strip around your finger), then find that value in the circumference column of the chart above and read across to find your US, UK, or EU size.
Is EU ring size the same as mm circumference?
Yes, approximately. EU size numbers closely match the inner circumference in mm. EU 52 means roughly 52mm inner circumference.
What ring size is 17mm inner diameter?
A 17mm inner diameter is approximately US 6.5 to 7, UK M½ to N, EU 53 to 54.
