Once you've measured your ring's inner diameter (or a finger's circumference) in millimetres, use this chart to find the matching size.
Ring sizing standards can vary slightly between manufacturers and countries. This chart uses commonly-published reference values — for an important purchase (like an engagement ring), it's worth double-checking with the specific jeweller's own size guide.
Diameter (mm) to size
| Inner diameter (mm) | India / UK | US |
|---|---|---|
| 14.9 | H | 4 |
| 15.3 | I | 5 |
| 15.7 | J | 5.5 |
| 16.1 | K | 6 |
| 16.5 | L | 6.5 |
| 16.9 | M | 7 |
| 17.3 | N | 7.5 |
| 17.7 | O | 8 |
| 18.1 | P | 8.5 |
| 18.5 | Q | 9 |
| 18.9 | R | 9.5 |
| 19.4 | S | 10 |
| 19.8 | T | 10.5 |
| 20.2 | U | 11 |
Circumference (mm) to diameter
If you measured around the finger (with string or paper) instead of an existing ring's inside edge, convert circumference to diameter first:
diameter = circumference ÷ π (≈ 3.14159)
For example, a 54mm circumference gives a diameter of about 17.2mm — close to size L (India/UK) or 6.5 (US).
How to measure accurately
- Measure at the base of the finger, at the time of day your fingers are their "normal" size (avoid right after exercise or in very cold weather, when size can shift slightly).
- If measuring an existing ring, measure the inside edge to edge, straight through the centre — not the outside.
- Take two readings and use the more consistent one.
No ruler or measuring tape handy? The free Screen Ruler works in mm right on your screen — calibrate it with a card first for an accurate reading. See our guide on checking ring size without a ring sizer for the full method.