SHOPPING · RING SIZE
Ring Size Calculator
Measure your ring size at home from finger circumference, a ring’s inside diameter or a card-calibrated circle on screen, then convert between US, UK, ISO, Japanese and Swiss sizes.
Measure your ring size
Example
A finger that measures 2.14 inches (54.4 mm) around is US 7, UK N½, ISO 54 and Japanese size 14. A ring with a 16 mm inside diameter is about 50.3 mm around: ISO 50 and Japanese size 10.
How the conversion works
Every ring size describes the inside of the ring. ISO 8653 uses the inside circumference in millimeters as the size. US and Canadian sizes follow inside circumference = 2.55 × size + 36.5 mm, in quarter steps. UK and Australian letter sizes follow the 1987 metric update, BS 6820: size C is 40 mm around and each letter adds 1.25 mm, with half sizes. Swiss and Italian sizes are the circumference minus 40. The tool converts your measurement to inside circumference, then shows the nearest size in each system, or both neighbors if you land close to the midpoint.
No official standard was found for Japanese and Korean sizes. The tool uses a common retailer conversion (size 1 = 13 mm inside diameter, about 1/3 mm per size), so a shop’s chart may differ by half to one size.
Size chart
| Japan | Diameter (mm) | Circumference (mm) | US | UK | ISO |
|---|---|---|---|---|---|
| 1 | 13 | 40.8 | 1¾ | C½ | 41 |
| 2 | 13.3 | 41.9 | 2 | D½ | 42 |
| 3 | 13.7 | 42.9 | 2½ | E½ | 43 |
| 4 | 14 | 44 | 3 | F | 44 |
| 5 | 14.3 | 45 | 3¼ | G | 45 |
| 6 | 14.7 | 46.1 | 3¾ | H | 46 |
| 7 | 15 | 47.1 | 4¼ | H½ | 47 |
| 8 | 15.3 | 48.2 | 4½ | I½ | 48 |
| 9 | 15.7 | 49.2 | 5 | J½ | 49 |
| 10 | 16 | 50.3 | 5½ | K | 50 |
| 11 | 16.3 | 51.3 | 5¾ | L | 51 |
| 12 | 16.7 | 52.4 | 6¼ | M | 52 |
| 13 | 17 | 53.4 | 6¾ | M½ | 53 |
| 14 | 17.3 | 54.5 | 7 | N½ | 54 |
| 15 | 17.7 | 55.5 | 7½ | O½ | 56 |
| 16 | 18 | 56.6 | 7¾ | P | 57 |
| 17 | 18.3 | 57.6 | 8¼ | Q | 58 |
| 18 | 18.7 | 58.6 | 8¾ | R | 59 |
| 19 | 19 | 59.7 | 9 | S | 60 |
| 20 | 19.3 | 60.7 | 9½ | S½ | 61 |
| 21 | 19.7 | 61.8 | 10 | T½ | 62 |
| 22 | 20 | 62.8 | 10¼ | U½ | 63 |
| 23 | 20.3 | 63.9 | 10¾ | V | 64 |
| 24 | 20.7 | 64.9 | 11¼ | W | 65 |
| 25 | 21 | 66 | 11½ | X | 66 |
| 26 | 21.3 | 67 | 12 | X½ | 67 |
| 27 | 21.7 | 68.1 | 12½ | Y½ | 68 |
| 28 | 22 | 69.1 | 12¾ | - | 69 |
| 29 | 22.3 | 70.2 | 13¼ | - | 70 |
| 30 | 22.7 | 71.2 | 13½ | - | 71 |
Home measurements are approximate. Fingers change with time of day and temperature, and band width changes how a ring feels. For an expensive ring, confirm with a jeweler’s ring sizer.
Frequently asked questions
How do I measure my ring size at home?
Wrap a strip of paper or string around the finger where the ring sits, mark the overlap and measure the length in millimeters or inches. Measuring the inside diameter of a ring that already fits is usually more accurate.
What is a US size 7 in UK and Japanese sizes?
US 7 is about 54.4 mm around the inside (17.3 mm across). That is UK N½, ISO 54 and, by the common retailer conversion, Japanese size 14.
What if I am between two sizes?
Choose the larger size for wide bands or larger knuckles and the smaller one for thin bands. Fingers swell in the evening and in warm weather, so measure more than once.
Is the on-screen method accurate?
It can be close once you calibrate with a card that is 85.6 mm wide, but zooming, rotating the screen or tilting the ring breaks it. Treat it as a guide and confirm with a jeweler.
More tools
Sources and last updated
- Wikipedia, Ring size: ISO 8653, the US and Canadian formula, UK BS 6820 and Swiss sizes (checked 2026-10-27).
- Japanese and Korean sizes: no official standard found; common retailer conversion.
- Calibration card: ISO/IEC 7810 ID-1, 85.60 × 53.98 mm.
- The “between two sizes” range (±8% of a half step) is a site default. Only the screen calibration is saved in this browser; your measurements are not stored.
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