Calculator
Enter a ring measurement in centimeters, convert it to millimeters, and match it against common size references.
Plotly graph
This chart maps common US ring sizes to circumference in millimeters and highlights your nearest matched point.
Example data table
These sample rows show how centimeter measurements become millimeter outputs and then align with common ring sizes.
| Sample input | Type | Converted mm | Estimated diameter | Estimated circumference | Nearest US size |
|---|---|---|---|---|---|
| 4.93 cm | Circumference | 49.30 mm | 15.70 mm | 49.30 mm | 5 |
| 5.44 cm | Circumference | 54.40 mm | 17.32 mm | 54.40 mm | 7 |
| 1.819 cm | Diameter | 18.19 mm | 18.19 mm | 57.15 mm | 8 |
| 1.984 cm | Diameter | 19.84 mm | 19.84 mm | 62.33 mm | 10 |
| 6.50 cm | Circumference | 65.00 mm | 20.69 mm | 65.00 mm | 11 |
Formula used
Core conversion
Millimeters = Centimeters × 10
This direct conversion changes your original ring measurement from centimeters into millimeters without changing the real physical size.
Ring geometry
Circumference = Diameter × π
Diameter = Circumference ÷ π
How to use this calculator
- Enter the ring measurement in centimeters.
- Choose whether the value is a circumference or diameter.
- Select your preferred fit style and decimal precision.
- Choose the size system you want highlighted first.
- Press Convert Ring Size to show the report above the form.
- Review the chart, matched sizes, and download the CSV or PDF report.
Ring size reference table
Use this table to compare common size systems. Circumference values are especially useful when converting measured string lengths.
| US | UK | EU | JP | Diameter (mm) | Circumference (mm) |
|---|---|---|---|---|---|
| 3 | F 1/2 | 44 | 4 | 14.07 | 44.20 |
| 3.5 | G 1/2 | 45 | 5 | 14.45 | 45.50 |
| 4 | H 1/2 | 47 | 7 | 14.86 | 46.80 |
| 4.5 | I 1/2 | 48 | 8 | 15.27 | 48.00 |
| 5 | J 1/2 | 49 | 9 | 15.70 | 49.30 |
| 5.5 | K 1/2 | 51 | 10 | 16.10 | 50.60 |
| 6 | L 1/2 | 52 | 12 | 16.51 | 51.90 |
| 6.5 | M 1/2 | 53 | 13 | 16.92 | 53.10 |
| 7 | N 1/2 | 54 | 14 | 17.35 | 54.40 |
| 7.5 | O 1/2 | 56 | 15 | 17.75 | 55.70 |
| 8 | P 1/2 | 57 | 16 | 18.19 | 57.00 |
| 8.5 | Q 1/2 | 58 | 17 | 18.53 | 58.30 |
| 9 | R 1/2 | 60 | 18 | 18.89 | 59.50 |
| 9.5 | S 1/2 | 61 | 19 | 19.41 | 61.00 |
| 10 | T 1/2 | 62 | 20 | 19.84 | 62.30 |
| 10.5 | U 1/2 | 63 | 22 | 20.20 | 63.50 |
| 11 | V 1/2 | 65 | 23 | 20.68 | 65.00 |
| 11.5 | W 1/2 | 66 | 24 | 21.08 | 66.30 |
| 12 | Y | 67 | 25 | 21.49 | 67.50 |
| 12.5 | Z | 69 | 26 | 21.89 | 68.80 |
| 13 | Z+1 | 70 | 27 | 22.33 | 70.20 |
FAQs
1. Why convert ring size from cm to mm?
Millimeters are the standard jewelry unit for precise ring sizing. Converting from centimeters removes ambiguity and makes matching against size charts much easier.
2. Should I enter circumference or diameter?
Enter whichever measurement you actually collected. Circumference works well with string methods, while diameter is best when measuring an existing ring’s inside width.
3. Does a wide band change the recommended size?
Yes. Wide bands often feel tighter, so many buyers prefer a slight comfort adjustment. This tool includes a comfort option to reflect that practical difference.
4. Is the nearest size always the best one?
Not always. If your result sits between standards, finger temperature, knuckle shape, and band width can influence whether you size up or down.
5. Can I use this for online ring shopping?
Yes. The calculator is helpful for comparing measurements before ordering online, especially when stores provide ring sizes by region instead of direct millimeter values.
6. What is the difference between EU and US ring sizes?
US sizes are numeric steps, while EU sizes are usually tied more closely to circumference values. This page shows both so you can compare systems quickly.
7. Why does my size change during the day?
Finger size can shift because of temperature, hydration, activity, and time of day. Measuring more than once usually gives a safer purchase decision.
8. Is this calculator a replacement for professional sizing?
No. It is excellent for planning and comparison, but a jeweler’s metal sizer remains the most reliable option for expensive custom or final-fit purchases.