Ring size chart
Enter a known measurement and read the size in all four systems, or browse the full chart from size 3 to 13 in quarter increments.
Convert an inner diameter or circumference into a ring size
Record the measurement in millimeters and select the nearest published row below. Convert inches to millimeters by multiplying by 25.4. US, UK and JP sizes use the shared reference chart; EU is rounded inner circumference. A dash marks an unavailable regional value.
Worked chart example: an inner diameter of 17.3 mm or circumference of 54.3 mm maps to US 7, UK N, EU 54 and JP 14 in the shared chart. These rounded dimensions illustrate a lookup, not a physical accuracy test or a guarantee of fit.
For a new measurement, use the free calibrated online ring sizer or A4 or US Letter printable ring sizer. Confirm the measured millimeters, intended finger and band width for your custom lab-grown diamond ring before production.
Reference sources: GIA: Ring size guide and chart; NIST: The inch is exactly 25.4 millimeters. See the calibration method and repeatability checks.
Full chart in quarter sizes
Inner diameter and circumference in millimeters.
| US / CA | UK / AU | EU / ISO | Asia / JP | Ø mm | mm |
|---|---|---|---|---|---|
| 3 | F | 44 | 4 | 14.1 | 44.3 |
| 3.25 | F½ | 45 | 5 | 14.3 | 44.9 |
| 3.5 | G | 46 | — | 14.5 | 45.6 |
| 3.75 | G½ | 46 | 6 | 14.7 | 46.2 |
| 4 | H | 47 | 7 | 14.9 | 46.8 |
| 4.25 | H½ | 47 | — | 15.1 | 47.4 |
| 4.5 | I | 48 | 8 | 15.3 | 48.1 |
| 4.75 | I½ | 49 | — | 15.5 | 48.7 |
| 5 | J | 49 | 9 | 15.7 | 49.3 |
| 5.25 | J½ | 50 | — | 15.9 | 50 |
| 5.5 | K | 51 | 10 | 16.1 | 50.6 |
| 5.75 | K½ | 51 | — | 16.3 | 51.2 |
| 6 | L | 52 | 11 | 16.5 | 51.8 |
| 6.25 | L½ | 53 | 12 | 16.7 | 52.5 |
| 6.5 | M | 53 | 13 | 16.9 | 53.1 |
| 6.75 | M½ | 54 | — | 17.1 | 53.7 |
| 7 | N | 54 | 14 | 17.3 | 54.3 |
| 7.25 | N½ | 55 | — | 17.5 | 55 |
| 7.5 | O | 56 | 15 | 17.7 | 55.6 |
| 7.75 | O½ | 56 | — | 17.9 | 56.2 |
| 8 | P | 57 | 16 | 18.1 | 56.9 |
| 8.25 | P½ | 58 | — | 18.3 | 57.5 |
| 8.5 | Q | 58 | 17 | 18.5 | 58.1 |
| 8.75 | Q½ | 59 | — | 18.8 | 59.1 |
| 9 | R | 60 | 18 | 19 | 59.7 |
| 9.25 | R½ | 60 | — | 19.2 | 60.3 |
| 9.5 | S | 61 | 19 | 19.4 | 60.9 |
| 9.75 | S½ | 62 | — | 19.6 | 61.6 |
| 10 | T | 62 | 20 | 19.8 | 62.2 |
| 10.25 | T½ | 63 | 21 | 20 | 62.8 |
| 10.5 | U | 64 | 22 | 20.2 | 63.5 |
| 10.75 | U½ | 64 | — | 20.4 | 64.1 |
| 11 | V | 65 | 23 | 20.6 | 64.7 |
| 11.25 | V½ | 65 | — | 20.8 | 65.3 |
| 11.5 | W | 66 | 24 | 21 | 66 |
| 11.75 | W½ | 67 | — | 21.2 | 66.6 |
| 12 | X | 67 | 25 | 21.4 | 67.2 |
| 12.25 | X½ | 68 | — | 21.6 | 67.9 |
| 12.5 | Y | 69 | 26 | 21.8 | 68.5 |
| 12.75 | Z | 69 | — | 22 | 69.1 |
| 13 | Z½ | 70 | 27 | 22.2 | 69.7 |
Conversion questions
How do I convert a diameter or circumference to US, UK, EU and JP ring sizes?
Measure the ring's inner diameter or the finger's circumference in millimeters, then use the nearest published row of this chart. For inches, multiply by 25.4 first. US, UK and JP values use the shared reference rows; EU is the rounded inner circumference in millimeters. A dash means that the reference row has no regional value, not a size inferred between entries.
What measurement range does this ring size converter support?
The converter accepts inner diameters from 14.1 to 22.2 mm or circumferences from 44.3 to 69.7 mm, covering US sizes 3–13 in quarter-size steps. It selects the nearest supported row and does not extrapolate outside that range. Quarter-size steps describe the chart, not guaranteed measurement accuracy.
Does a converted ring size confirm the fit for production?
No. Conversion maps a measurement to a chart row; it does not check calibration, paper tension, knuckle passage or band width. Repeat the measurement three times, cross-check with a second method or professional fitting, and confirm your intended finger and band width before production.
How do I read this conversion chart?
The US/CA column is the primary reference. Diameter and circumference in millimeters are physical measurements that are identical across every system; UK letters and EU/JP numbers are standardized conversions of those measurements.
What is the difference between diameter and circumference?
Diameter is the straight-line distance between two opposite inner edges of a ring. Circumference is the full inner perimeter, equal to the diameter multiplied by π (about 3.14).
Why does my EU size sometimes differ by one number?
EU sizes round the circumference to the nearest millimeter, and different houses round differently. Always confirm against the chart of the brand you are buying from.