Ring size conversion
Updated: 14 July 2026
The European ring size is the inside circumference in millimeters, and every other number follows from it. Size 54 is 54 mm circumference, 17.2 mm diameter, about US 7 and UK N. Here are the calculations and the full chart.
Converting between diameter and circumference
Pi links the two:
- Diameter times 3.1416 gives the circumference, which is the European size.
- Circumference divided by 3.1416 gives the diameter.
Example: 17.2 mm times 3.1416 is about 54 mm, so size 54.
From EU to US and UK
There is no clean formula for US and UK sizes. The systems start at different points and run in different steps. One US size up is about 0.8 mm more inside diameter, one UK letter up is about 1.3 mm more circumference.
Rather than calculating, read the matching value off the chart. It is in every row.
Conversion chart
| Size (circumference) | Diameter | US | UK |
|---|---|---|---|
| 44 mm | 14 mm | US 3 | UK F |
| 45 mm | 14.3 mm | US 3.5 | UK G |
| 46 mm | 14.6 mm | US 3.5 | UK G |
| 47 mm | 15 mm | US 4 | UK H |
| 48 mm | 15.3 mm | US 4.5 | UK I |
| 49 mm | 15.6 mm | US 5 | UK J |
| 50 mm | 15.9 mm | US 5.5 | UK K |
| 51 mm | 16.2 mm | US 5.5 | UK L |
| 52 mm | 16.6 mm | US 6 | UK M |
| 53 mm | 16.9 mm | US 6.5 | UK M |
| 54 mm | 17.2 mm | US 7 | UK N |
| 55 mm | 17.5 mm | US 7 | UK O |
| 56 mm | 17.8 mm | US 7.5 | UK P |
| 57 mm | 18.1 mm | US 8 | UK Q |
| 58 mm | 18.5 mm | US 8.5 | UK Q |
| 59 mm | 18.8 mm | US 9 | UK R |
| 60 mm | 19.1 mm | US 9 | UK S |
| 61 mm | 19.4 mm | US 9.5 | UK T |
| 62 mm | 19.7 mm | US 10 | UK T |
| 63 mm | 20.1 mm | US 10.5 | UK U |
| 64 mm | 20.4 mm | US 11 | UK V |
| 65 mm | 20.7 mm | US 11 | UK W |
| 66 mm | 21 mm | US 11.5 | UK X |
| 67 mm | 21.3 mm | US 12 | UK X |
| 68 mm | 21.6 mm | US 12.5 | UK Y |
| 69 mm | 22 mm | US 12.5 | UK Z |
| 70 mm | 22.3 mm | US 13 | UK above Z |
| 71 mm | 22.6 mm | US 13.5 | UK above Z |
| 72 mm | 22.9 mm | US 14 | UK above Z |
Measure first
These are the reliable ways to measure:
- Take a ring that already fits the right finger, lay it on a ruler and measure the inside diameter from edge to edge.
- Or use a ring sizer, a set of sample rings you try on one by one.
- Or have a jeweler measure your finger.
The string method (wrap a string around the finger and measure its length) is a fallback for when none of the above is possible. It gives you a rough estimate. Measure the finger you want to wear the ring on and remember that the ring also has to pass your knuckle.
Common questions
How do I convert my ring size to US?
Measure the inside circumference in millimeters and read the US size off the chart. Size 54, for example, is about US 7.
Can I calculate the ring size from the diameter?
Yes. Diameter times 3.1416 gives the circumference, which is the European size. 17.2 mm gives 54 mm, so size 54.
Why are US ring sizes half numbers?
The US scale uses half steps. A whole step is about 0.8 mm of inside diameter, a half step about 0.4 mm.