Ring Size to mm Conversion Chart
US ring sizes 3–13.5 converted to inner diameter in millimeters. Quick-reference chart for buying rings from international sellers or measuring at home.
For a custom value not in this chart, use the interactive US / Canada Ring Size to Diameter (mm) converter.
| US / Canada Ring Size (US) | Diameter (mm) (mm ∅) |
|---|---|
| 3 US | 14.016 mm ∅ |
| 3.5 US | 14.432 mm ∅ |
| 4 US | 14.848 mm ∅ |
| 4.5 US | 15.264 mm ∅ |
| 5 US | 15.68 mm ∅ |
| 5.5 US | 16.096 mm ∅ |
| 6 US | 16.512 mm ∅ |
| 6.5 US | 16.928 mm ∅ |
| 7 US | 17.344 mm ∅ |
| 7.5 US | 17.76 mm ∅ |
| 8 US | 18.176 mm ∅ |
| 8.5 US | 18.592 mm ∅ |
| 9 US | 19.008 mm ∅ |
| 9.5 US | 19.424 mm ∅ |
| 10 US | 19.84 mm ∅ |
| 10.5 US | 20.256 mm ∅ |
| 11 US | 20.672 mm ∅ |
| 11.5 US | 21.088 mm ∅ |
| 12 US | 21.504 mm ∅ |
| 12.5 US | 21.92 mm ∅ |
| 13 US | 22.336 mm ∅ |
| 13.5 US | 22.752 mm ∅ |
Conversion Formula
inner diameter (mm) ≈ 11.52 + (US size × 0.832) Example: US 7 → 11.52 + 5.82 = 17.34 mm Each full US size ≈ 0.83 mm of diameter Each half size ≈ 0.4 mm Related measurements: inner circumference (mm) = diameter × π US 7 → 17.34 mm ∅ → 54.5 mm circumference (≈ EU size 54–55) Common anchors: US 6 = 16.5 mm ∅ (common women's size) US 10 = 19.8 mm ∅ (common men's size)
Frequently Asked Questions
Is a ring size in mm the diameter or the circumference?
Both conventions exist, so check which one a seller means. This chart shows inner diameter — the straight-line distance across the inside of the band. EU/ISO ring sizes use inner circumference instead, which is the diameter multiplied by π (about 3.14).
What ring size is 17 mm?
A 17 mm inner diameter falls between US size 6.5 (16.9 mm) and US 7 (17.3 mm) — closer to 6.5. When a measurement lands between sizes, jewelers generally recommend the larger one.
How many mm apart are consecutive ring sizes?
Each full US size adds about 0.83 mm of inner diameter (about 2.6 mm of circumference); each half size adds about 0.4 mm. That's why precise measurement matters — the difference between a comfortable fit and a tight one is under half a millimeter.
Can I measure a ring's mm size with a regular ruler?
Yes, for a rough reading: measure straight across the inside of the band at its widest point, edge to edge of the metal's interior. A caliper is far more accurate, since reading a ruler to the nearest 0.4 mm — one half size — is difficult by eye.
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Need a custom value?
This chart covers common values. For any number, use the interactive converter.
Use the interactive converter →All conversion values are calculated using internationally recognized conversion factors. Results are provided for general informational purposes — verify critical values independently. Read our full disclaimer.