Ring Size Conversion Chart
Convert US numeric, UK alphabetical and European ring sizes, with inside diameter and circumference in millimeters.
A US ring size converts to millimeters as inside diameter = 11.54 + 0.8128 × size. A US 6 is 16.5 mm across, a UK M, and an EU 51.9 since the European size is simply the inside circumference in millimeters.
By Piero Nanni · Last checked
Matching rows are highlighted in the tables below.
Ring sizing is the one entry on this page with a real formula behind it. The US scale is linear in diameter: each whole size adds 0.8128 mm, which is exactly one thirty-second of an inch. The European system skips the labels entirely and states the inside circumference in millimeters, so an EU 52 ring measures 52 mm around the inside.
The UK system uses letters, running from A upward in half-letter steps, and is shared with Australia, New Zealand, Ireland and South Africa. Because letters and numbers do not divide the same way, US half sizes and UK half letters do not line up exactly, and most conversion tables round in slightly different places. Where the table below shows a half letter, the nearest whole letter is usually the safer buy.
Ring size conversion
Diameter and circumference are the inside measurements of the band.
| US | UK | EU (circumference mm) | Diameter (mm) |
|---|---|---|---|
| 3 | F | 44.2 | 14.1 |
| 3.5 | G | 45.5 | 14.5 |
| 4 | H | 46.8 | 14.9 |
| 4.5 | I | 48.0 | 15.3 |
| 5 | J½ | 49.3 | 15.7 |
| 5.5 | L | 50.6 | 16.1 |
| 6 | M | 51.9 | 16.5 |
| 6.5 | N | 53.1 | 16.9 |
| 7 | O | 54.4 | 17.3 |
| 7.5 | P | 55.7 | 17.7 |
| 8 | Q | 57.0 | 18.1 |
| 8.5 | Q½ | 58.3 | 18.5 |
| 9 | R½ | 59.5 | 19.0 |
| 9.5 | S½ | 60.8 | 19.4 |
| 10 | T½ | 62.1 | 19.8 |
| 10.5 | U½ | 63.4 | 20.2 |
| 11 | V½ | 64.6 | 20.6 |
| 11.5 | W | 65.9 | 21.0 |
| 12 | X | 67.2 | 21.4 |
| 12.5 | Y | 68.5 | 21.8 |
| 13 | Z | 69.7 | 22.2 |
The formula behind the US scale
Inside diameter in millimeters is 11.54 + 0.8128 × the US size. The 0.8128 is one thirty-second of an inch, so a full US size is a thirty-second of an inch of diameter and a half size is a sixty-fourth.
Circumference follows as π times the diameter, which is what the European number states directly. That makes EU sizing the most transferable of the three: a jeweller anywhere can check it with a millimeter gauge and no conversion table.
How to measure a finger without a sizer
Wrap a strip of paper around the base of the finger, mark where it overlaps, and measure the length in millimeters. That length is the inside circumference, which is the EU size, and the table converts it to US and UK from there.
Do not use string; it stretches, and it will read a size or more too large. Do not measure over a knuckle unless the knuckle is much larger than the base of the finger, in which case size to slide over the knuckle and expect the ring to spin.
When to measure
Fingers change size through the day and through the year. They are smallest in the morning and in cold weather, and can swell by up to a full size in heat or after exercise or salt. Measure warm and at rest, in the late afternoon.
Wide bands fit tighter than narrow ones at the same nominal size, so add a quarter to a half size for a band over about 6 mm wide. Any jeweller will confirm this before cutting, and resizing a wide band afterwards is expensive.
Frequently Asked Questions
What is a US size 7 ring in UK?
A US 7 is a UK O, with an inside diameter of 17.3 mm.
What is EU ring size 52 in US?
EU 52 is a US 6, since the EU number is the inside circumference in millimeters.
How do I convert ring diameter to US size?
Subtract 11.54 from the diameter in millimeters, then divide by 0.8128.
What is the average ring size?
Around US 6 to 6.5 for women and US 9 to 10 for men, though this varies widely and is a poor substitute for measuring.
Should I size up for a wide band?
Yes. Add a quarter to a half size for bands wider than about 6 mm.