Converter
Export & Sample Data
| # | Mode | Input (cm) | Input (mm) | Diameter (mm) | Circumference (mm) | US | UK | EU |
|---|---|---|---|---|---|---|---|---|
| 1 | Diameter | 1.65 | 16.50 | 16.50 | 51.84 | 6 | L | 52 |
| 2 | Circumference | 5.70 | 57.00 | 18.14 | 57.00 | 8 | P | 57 |
| 3 | Diameter | 2.00 | 20.00 | 20.00 | 62.83 | 10 | T | 63 |
Formula Used
- Unit conversion: mm = cm × 10 and cm = mm ÷ 10.
- Geometry: circumference = π × diameter, diameter = circumference ÷ π.
- EU (ISO) size ≈ inner circumference in millimeters (rounded to nearest whole).
- US/UK sizes are matched by nearest inner diameter to standard lookup tables (approximate, varies by manufacturer).
How to Use This Calculator
- Select Diameter if you measured the inside edge‑to‑edge of the ring; select Circumference if you wrapped a string around the inside.
- Enter the measured value in centimeters. The millimeters field fills automatically.
- Click Compute to see the interpreted diameter, circumference, and nearest US/UK/EU sizes.
- Click Add to Table to append the result row; repeat for multiple rings or scenarios.
- Use Download CSV or Download PDF to export the table for printing or sharing.
Frequently Asked Questions
In‑Depth Guide: Choosing the Right Ring Size (≈500 words)
Getting ring size right is equal parts measurement and judgment. Your tools—calipers, a paper strip, or a flexible tailor’s tape—can all work when used carefully. Start by deciding whether you can measure the inside diameter directly or whether circumference suits your situation better. If you already have a ring that fits the intended finger, measure its inside diameter across the widest interior points. If you do not, wrap a thin paper strip around the base of the finger, mark the overlap, and measure that length as inner circumference.
Because a circle’s circumference equals π times its diameter, the two measurements are interchangeable. This calculator accepts either, but uses millimeters internally for consistency. Converting centimeters to millimeters multiplies by ten, which reduces rounding loss; you will see both diameter and circumference reported to two decimal places for readability. While the EU system is straightforward—your ring size is essentially the inner circumference in millimeters—the US and UK systems rely on standardized tables mapped to inner diameter. Brands sometimes deviate slightly due to rounding, mandrel wear, or comfort‑fit shaping. Treat the provided US and UK sizes as high‑quality estimates rather than absolute truth.
Several human factors influence fit. Fingers swell with temperature, hydration, and activity. Many people measure a half size larger in summer than in winter, and a quarter size larger in the evening than in the morning. Knuckle prominence also matters: if the knuckle is significantly larger than the base of the finger, size for the knuckle but choose a comfort‑fit profile so the ring feels secure once on. Band width changes perception too; wider bands feel tighter and may warrant sizing up by a quarter to a half step. When in doubt, try sample bands with similar width and profile.
For online purchases, create a quick sizing kit: a printable strip with a millimeter scale, a card to record multiple measurements across two days, and notes on band width and temperature. Use this calculator to convert, log entries into the table, and export to CSV or PDF for sharing with a jeweler or keeping in your records. If your result sits between sizes, prioritize comfort and safety—slightly loose is easier to manage than a ring that will not pass the knuckle. For valuable pieces, confirm using a jeweler’s professional sizing set before finalizing. Accurate, repeatable measurements plus informed judgment will deliver a ring that feels right every day.
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