Ring Size mm to US and UK
Millimetres are the most reliable way to size a ring, because every national system is derived from the same physical measurement.
Quick answer
A 17.3 mm inside diameter (54.3 mm circumference) is a US 7 and a UK N½. Divide any circumference by 3.14 to get the diameter.
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Inside circumference, mm
Inside
Inside
Inside
Inside
Matched inside diameter 17.30 mm · circumference 54.35 mm
Conversion table
| Inside diameter | Circumference | US | UK |
|---|---|---|---|
| 14.00 mm | 44.0 mm | 3 | F |
| 14.40 mm | 45.2 mm | 3.5 | G |
| 14.80 mm | 46.5 mm | 4 | H |
| 15.20 mm | 47.8 mm | 4.5 | I |
| 15.60 mm | 49.0 mm | 5 | J½ |
| 16.00 mm | 50.3 mm | 5.5 | K½ |
| 16.45 mm | 51.7 mm | 6 | L½ |
| 16.90 mm | 53.1 mm | 6.5 | M½ |
| 17.30 mm | 54.3 mm | 7 | N½ |
| 17.70 mm | 55.6 mm | 7.5 | O½ |
| 18.10 mm | 56.9 mm | 8 | P½ |
| 18.50 mm | 58.1 mm | 8.5 | Q½ |
| 19.00 mm | 59.7 mm | 9 | R½ |
| 19.40 mm | 60.9 mm | 9.5 | S½ |
| 19.80 mm | 62.2 mm | 10 | T½ |
| 20.20 mm | 63.5 mm | 10.5 | U½ |
| 20.60 mm | 64.7 mm | 11 | V½ |
| 21.00 mm | 66.0 mm | 11.5 | W½ |
| 21.40 mm | 67.2 mm | 12 | X½ |
| 21.80 mm | 68.5 mm | 12.5 | Z |
| 22.20 mm | 69.7 mm | 13 | Z+1 |
Diameter or circumference?
Inside diameter is the straight-line measurement across the middle of the ring. Inside circumference is the distance all the way around. Circumference = diameter × 3.14159.
If you measured your finger with string you have a circumference. If you measured an existing ring with a ruler you have a diameter.
Common millimetre conversions
- 15.6 mm = US 5 = UK J½
- 16.45 mm = US 6 = UK L½
- 17.3 mm = US 7 = UK N½
- 18.1 mm = US 8 = UK P½
- 19.0 mm = US 9 = UK R½
Rounding rules
If your measurement falls between two rows, round up rather than down, especially for a wide band. Half a millimetre of diameter is roughly half a US size.
Frequently asked questions
What ring size is 17 mm?
A 17 mm inside diameter sits between a US 6½ and a US 7 — most people order the US 7 (UK N½).
How do I turn circumference into diameter?
Divide the circumference by 3.14159. A 54 mm circumference is a 17.2 mm diameter.
Keep reading
For an expensive ring, confirm the size with a jeweller before ordering — see the ring size FAQs for resizing advice.