Estimate a nominal US ring size using the GIA international chart, or look up dimensions from a US size. Supports mm, cm and in, UK chart entries, a rounded circumference-size estimate and paper strip length. Fit preference and band width do not apply unsupported automatic corrections; confirm fit with a jeweler.
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Cm To Ring Size Formula
Convert measurements to millimeters, then use the GIA international sizing chart. There is no universal linear equation converting circumference to a US size.
Variables:
- US size is the nominal GIA chart size (supported range 3โ13.5)
- C_cm is the measured finger circumference in centimeters; C_mm is circumference in millimeters
For an existing ring, match its inside diameter to the nearest GIA chart entry. For finger circumference between entries, this calculator chooses the next larger entry. Chart dimensions are rounded and fit depends on knuckle size, band width and measurement conditions; use a physical ring sizer or jeweler to confirm.
| Ring Size (US) | Inside Diameter (mm) | Inside Diameter (cm) | Inside Diameter (in) |
|---|---|---|---|
| 3 | 14.10 | 1.410 | 0.555 |
| 3.5 | 14.50 | 1.450 | 0.571 |
| 4 | 14.90 | 1.490 | 0.587 |
| 4.5 | 15.30 | 1.530 | 0.602 |
| 5 | 15.70 | 1.570 | 0.618 |
| 5.5 | 16.10 | 1.610 | 0.634 |
| 6 | 16.50 | 1.650 | 0.650 |
| 6.5 | 16.90 | 1.690 | 0.665 |
| 7 | 17.30 | 1.730 | 0.681 |
| 7.5 | 17.70 | 1.770 | 0.697 |
| 8 | 18.10 | 1.810 | 0.713 |
| 8.5 | 18.50 | 1.850 | 0.728 |
| 9 | 19.00 | 1.900 | 0.748 |
| 9.5 | 19.40 | 1.940 | 0.764 |
| 10 | 19.80 | 1.980 | 0.780 |
| 10.5 | 20.20 | 2.020 | 0.795 |
| 11 | 20.60 | 2.060 | 0.811 |
| 11.5 | 21.00 | 2.100 | 0.827 |
| 12 | 21.40 | 2.140 | 0.843 |
| 13 | 22.20 | 2.220 | 0.874 |
| Nominal GIA chart diameters; cm and inch columns are rounded conversions. 1 inch = 25.4 mm. Verify the sellerโs chart and fit; circumference-based ISO size is a different designation. | |||
What is a Ring Size?
Ring size is a measurement used to denote the circumference or diameter of a ring. It is typically measured in millimeters or inches and is used to ensure that a ring fits comfortably on a person's finger. Ring sizes can vary based on regional standards, but the most common systems include the US, UK, and European sizing charts. Knowing your ring size is essential when purchasing rings, especially for special occasions such as engagements or weddings.
How to Calculate Ring Size?
The following steps outline how to calculate the ring size.
- First, measure an existing ringโs inside diameter or obtain a finger measurement with a physical ring sizer.
- Next, select the measurement mode and its unit; multiply cm by 10 to convert to mm.
- Match the appropriate diameter or circumference column in the GIA chart; do not extrapolate beyond its supported range.
- Finally, read the estimated US size and confirm fit with a jeweler. Paper or string measurements are approximate.
- After inserting the values and calculating the result, check your answer with the calculator above.
Example Problem :
Use the following variables as an example problem to test your knowledge.
Finger Circumference (C) = 5.5 cm
55 mm finger circumference matches US 7.25 in the GIA chart. This is an estimate; it is not US 2.5 and does not guarantee fit.
