Ring size in millimetres & inches
Millimetres (and inches) are the universal language of ring sizing, every national system is really just a label on a physical measurement. Convert a measurement to a size, or a size to mm and inches, below.
Measurement → ring size
Pick Circumference or Diameter, switch the unit toggle to mm or in, and enter your measurement.
Diameter vs circumference
Inner diameter is the distance straight across the inside of the ring. Inner circumference is the distance all the way around the inside. They are two views of the same ring:
circumference = diameter × 3.1416 · 1 inch = 25.4 mm
A ring measuring 17.2 mm (0.68 in) across is 54 mm around, an EU 54 / US 7. When you measure at home, measuring across an existing ring is usually the most reliable. Working in inches? The converter’s mm/in toggle takes either.
Ring size to mm & inches chart
Common sizes with their inner circumference and diameter in both millimetres and inches.
| USUS / Canada | UKUK / AU | EUEU / ISO | mmCircumference | inCircumference | mmDiameter | inDiameter |
|---|---|---|---|---|---|---|
| 4 | H½ | 47 | 46.7 | 1.84 | 14.9 | 0.587 |
| 4½ | I½ | 48 | 48.0 | 1.89 | 15.3 | 0.602 |
| 5 | J½ | 49 | 49.3 | 1.94 | 15.7 | 0.618 |
| 5½ | K½ | 51 | 50.5 | 1.99 | 16.1 | 0.634 |
| 6 | L½ | 52 | 51.8 | 2.04 | 16.5 | 0.650 |
| 6½ | M½ | 53 | 53.1 | 2.09 | 16.9 | 0.665 |
| 7 | N½ | 54 | 54.4 | 2.14 | 17.3 | 0.681 |
| 7½ | O½ | 56 | 55.6 | 2.19 | 17.7 | 0.697 |
| 8 | P½ | 57 | 56.9 | 2.24 | 18.1 | 0.713 |
| 8½ | Q½ | 58 | 58.2 | 2.29 | 18.5 | 0.728 |
| 9 | R½ | 59 | 59.5 | 2.34 | 18.9 | 0.744 |
| 9½ | S½ | 61 | 60.7 | 2.39 | 19.3 | 0.760 |
| 10 | T½ | 62 | 62.0 | 2.44 | 19.7 | 0.776 |
| 10½ | U½ | 63 | 63.3 | 2.49 | 20.1 | 0.791 |
| 11 | V½ | 65 | 64.6 | 2.54 | 20.5 | 0.807 |
| 11½ | W½ | 66 | 65.8 | 2.59 | 21.0 | 0.827 |
| 12 | X½ | 67 | 67.1 | 2.64 | 21.4 | 0.843 |
Frequently asked questions
What is my ring size in mm?
Your ring size in millimetres is the inner circumference of the band. For example, a US 7 is about 54.4 mm around and 17.3 mm across. Enter any size in the converter to see both figures.
What is my ring size in inches?
In inches, a US 7 is about 2.14 inches around the inside (circumference) and 0.68 inches across (diameter). Since 1 inch = 25.4 mm, you can measure in either unit, the converter’s mm/in toggle accepts both and shows the result in millimetres and inches.
Is ring size diameter or circumference?
Both are used. The inner diameter is the straight-line distance across the inside; the inner circumference is the full distance around. They are linked by circumference = diameter × π (3.1416). EU/ISO sizes use circumference; US and Japan are based on diameter.
How do I convert mm to a ring size?
If you measured around the finger, that is circumference, enter it directly. If you measured across a ring, that is diameter, the converter multiplies by π for you. Either way you get US, UK, EU and Japan sizes instantly.
See also the full ring size chart or how to measure at home.