Ring Size Converter
A US size 7 ring has an inside circumference of 54.3 mm and an inside diameter of 17.3 mm. That is about ISO/EU size 54 and UK size N½.
How to use
- Pick what you know: US size, UK letter size, EU (ISO) size, inside diameter or inside circumference in millimeters.
- Enter the value. The other sizes appear at once.
- To find your own size, measure the inside diameter of a ring that fits, or wrap a strip of paper around your finger and measure the circumference. Measure at the end of the day and compare with the chart below.
How it works
ISO 8653 defines ring size by the inside circumference of the ring in millimeters. The EU (France, Germany and others) sizes follow it: an ISO size of 54 is a circumference of 54 mm. The inside diameter is the circumference divided by pi, so ISO 54 is 17.2 mm across.
US sizes count in steps of about 0.8128 mm of diameter, which is 2.55 mm of circumference: circumference = 2.55 x US size + 36.5 mm. A US 7 is 2.55 x 7 + 36.5 = 54.35 mm, a diameter of 17.3 mm.
UK sizes use letters, starting at A for 37.5 mm of circumference with one letter equal to 1.25 mm and half sizes in between. US 7 falls at N½. Sizes are rounded to the nearest quarter (US) or half (UK) size.
Charts from different makers can differ by a fraction of a size, and a wide band fits tighter than a thin one, so size up by a quarter to a half size for wide bands. Fingers also swell with heat and in the evening, so measure at the end of the day.
Ring size chart
| US | UK | EU / ISO | Circumference (mm) | Diameter (mm) |
|---|---|---|---|---|
| 3 | F½ | 44 | 44.1 | 14.1 |
| 4 | H½ | 47 | 46.7 | 14.9 |
| 5 | J½ | 49 | 49.3 | 15.7 |
| 6 | L½ | 52 | 51.8 | 16.5 |
| 7 | N½ | 54 | 54.3 | 17.3 |
| 8 | P½ | 57 | 56.9 | 18.1 |
| 9 | R½ | 59 | 59.5 | 18.9 |
| 10 | T½ | 62 | 62.0 | 19.7 |
| 11 | V½ | 65 | 64.5 | 20.5 |
| 12 | X½ | 67 | 67.1 | 21.4 |
Sources
ISO 8653:2016, "Jewellery and precious metals: ring sizes", defines sizes by inside circumference in millimeters. US formula (circumference = 2.55 x size + 36.5 mm, diameter = 0.8128 x size + 11.63 mm) as published in the Wikipedia article "Ring size". The UK letter scale (A at 37.5 mm, 1.25 mm per letter) is the usual jewelers' convention and matches common charts; check the maker's own chart for an exact fit.
FAQ
How do I measure my ring size at home?
Wrap a thin strip of paper or string around the base of the finger, mark where it overlaps and measure the length in mm. That is your circumference. Or measure the inside diameter of a ring that fits. Repeat a couple of times at different times of day.
Is EU ring size the same as ISO size?
Yes for most of Europe. ISO 8653 sizes equal the inside circumference in mm, which is the same number used by the common continental European scale. Some countries, such as Japan, use their own scales, which are not converted here.
Do half sizes exist?
Yes. US sizes come in half and sometimes quarter sizes, and UK sizes in half letters. The tool rounds to the nearest quarter US size and half UK size.
Should I size up for a wide band?
Often yes. Wide bands (6 mm and over) fit more snugly, so many jewelers recommend going up by a quarter to a half size.