cm → Ring Size Converter
Take a circumference measurement in centimeters, and we’ll translate it to both US and UK ring sizes along with exact millimeter values.
Wrap a measuring tape around the base of your finger where the ring will sit.
Measurement: 5.40 cm = 54.0 mm circumference
Inside diameter: 17.19 mm
Closest match
US 7 • UK N 1/2
Reference circumference: 54.4 mm (difference ±0.40 mm)
How to Use This Converter
Measure circumference in centimeters
Use a flexible tape or string; mark where it overlaps and measure the length in cm.
Convert instantly
Enter the cm value to see millimeters and the nearest US/UK ring size.
Verify fit
Because hands swell/shrink, confirm by trying a physical ring or using a jeweler’s sizing kit.
Reference Ring Size Chart
| US Size | UK Size | Diameter (mm) | Circumference (mm) |
|---|---|---|---|
| 4 | H 1/2 | 14.8 | 46.5 |
| 4.5 | I 1/2 | 15.2 | 47.8 |
| 5 | J 1/2 | 15.7 | 49.3 |
| 5.5 | K 1/2 | 16.1 | 50.6 |
| 6 | L 1/2 | 16.5 | 51.9 |
| 6.5 | M 1/2 | 16.9 | 53.1 |
| 7 | N 1/2 | 17.3 | 54.4 |
| 7.5 | O 1/2 | 17.7 | 55.6 |
| 8 | P 1/2 | 18.2 | 57.2 |
| 8.5 | Q 1/2 | 18.6 | 58.4 |
| 9 | R 1/2 | 19.0 | 59.8 |
| 9.5 | S 1/2 | 19.4 | 60.9 |
| 10 | T 1/2 | 19.8 | 62.3 |
| 10.5 | U 1/2 | 20.2 | 63.5 |
| 11 | V 1/2 | 20.6 | 64.8 |
| 11.5 | W 1/2 | 21.0 | 66.0 |
| 12 | Y | 21.4 | 67.5 |
| 12.5 | Z | 21.8 | 68.7 |
| 13 | Z 1/2 | 22.2 | 69.9 |
Chart values follow common jeweler references. Regional sizing systems can vary slightly; always confirm before ordering.
Formula
Circumference (mm) = Circumference (cm) × 10
Diameter (mm) = Circumference (mm) ÷ π
Nearest Ring Size = Closest circumference entry in size table
Example: 5.4 cm → 54.0 mm circumference → closest to US 7 (54.4 mm).
Tips for Accurate Sizing
- Measure the finger at different times—morning vs evening—to find an average.
- For wider bands (>6 mm), size up slightly to accommodate limited flex.
- When in doubt, visit a jeweler for confirmation before ordering custom pieces.
- Consider seasonal changes; fingers can swell in summer and shrink in winter.
Frequently Asked Questions
Why do I need both circumference and diameter?
Some sizing charts reference diameter while others use circumference; we provide both for flexibility.
What if my measurement is between sizes?
Round to the nearest 0.5 or 0.25 size, or ask a jeweler to custom-fit. The helper note indicates if you’re between sizes.
Can I convert to EU or Asian ring sizes?
Use the provided mm values with region-specific charts. This tool focuses on US/UK conversions.
Is this suitable for printable ring sizers?
Yes—measure from the printable tool in cm, then convert here to confirm millimeter-based sizing.