Ring sizes look unrelated from country to country: a US 7 is an N½ in Britain, a 54 or 55 in much of Europe and a 14 in Japan. They are all describing the same thing, the size of the hole in the ring, so they convert neatly through millimeters. This converter takes any of those systems, or a direct measurement, and shows the rest.
How to use the ring size converter
- Choose what you know: a US, UK, EU or Japanese size, the inside diameter of a ring that fits, or your finger circumference.
- Enter the size or measurement. UK sizes accept letters such as M, N½ or Z+1; measurements accept millimeters, centimeters or inches.
- Read the equivalent sizes, inside diameter and circumference. The chart marks the row closest to your result.
Ring size formulas
The US formula reflects the traditional American scale, where each size step is 0.032 inch of diameter. UK letters follow British Standard BS 6820, with half sizes between letters, and continue past Z as Z+1, Z+2 and so on.
Worked example
You wear a US size 7:
Diameter = 11.63 + 0.8128 × 7 = 17.32 mm (0.682 in)
Circumference = π × 17.32 = 54.4 mm, so the EU/ISO size is about 54½ (sold as 54 or 55)
UK: (54.4 − 37.5) ÷ 1.25 = 13.5 letter steps past A → N½. Japan: 54.4 − 40 ≈ 14.
Ring size chart
| US | UK / AU | EU / ISO | Japan | Diameter (mm) | Circumference (mm) |
|---|---|---|---|---|---|
| 3 | F½ | 44 | 4 | 14.1 | 44.2 |
| 4 | H½ | 47 | 7 | 14.9 | 46.8 |
| 5 | J½ | 49.5 | 9 | 15.7 | 49.3 |
| 5½ | K½ | 50.5 | 11 | 16.1 | 50.6 |
| 6 | L½ | 52 | 12 | 16.5 | 51.9 |
| 6½ | M½ | 53 | 13 | 16.9 | 53.1 |
| 7 | N½ | 54.5 | 14 | 17.3 | 54.4 |
| 7½ | O½ | 55.5 | 16 | 17.7 | 55.7 |
| 8 | P½ | 57 | 17 | 18.1 | 57.0 |
| 8½ | Q½ | 58 | 18 | 18.5 | 58.2 |
| 9 | R½ | 59.5 | 20 | 18.9 | 59.5 |
| 9½ | S½ | 61 | 21 | 19.4 | 60.8 |
| 10 | T½ | 62 | 22 | 19.8 | 62.1 |
| 11 | V½ | 64.5 | 25 | 20.6 | 64.6 |
| 12 | X½ | 67 | 27 | 21.4 | 67.2 |
| 13 | Z+1 | 69.5 | 30 | 22.2 | 69.7 |
The converter also handles quarter sizes and values outside this range.
Measuring accurately
Use a ring that already fits
Measuring the inside diameter of a well-fitting ring on the same finger is the most reliable home method. Use a ruler with millimeter marks or calipers, measuring straight across the widest part of the opening.
Or measure the finger itself
Paper strips and string can stretch or be pulled too tight, so measure two or three times. The ring has to pass over the knuckle, so if your knuckle is much larger than the base of your finger, choose a size between the two measurements.
Measure at the right time
Fingers swell in the heat, after exercise, and late in the day, and shrink in the cold. Measure at room temperature in the evening for the most typical size, and remember that dominant hands are often slightly larger.
Brands and jewelers sometimes run a little large or small, so confirm with the jeweler’s own sizer before ordering a ring that cannot be resized. For other sizing questions, try the shoe size converter or the clothing size converter; for measurements, the millimeters to inches converter helps.
Frequently asked questions
How do I measure my ring size at home?
Wrap a strip of paper or string snugly around the base of your finger, mark where it overlaps, and measure the length in millimeters; that is your circumference. Or measure the inside diameter of a ring that fits well, edge to edge across the middle.
What is the most common ring size?
In the US, women's ring sizes are commonly around 6 to 7 and men's around 9 to 10. A US 7 has an inside diameter of about 17.3 mm.
How do US ring sizes relate to millimeters?
Each whole US size adds 0.032 inch (0.8128 mm) to the inside diameter, starting from 0.458 inch (11.63 mm) at size 0. So a size 6 is about 16.5 mm and a size 10 about 19.8 mm across.
What is a European ring size?
Most of Europe uses the ISO 8653 system, where the size number is the inside circumference in millimeters. A ring with a 54 mm inner circumference is a size 54, close to a US 7.
Should I size up for a wide band?
Often, yes. Bands wider than about 6 mm cover more of the finger and feel tighter, so many jewelers suggest going up a quarter to half size. Comfort-fit bands with a rounded inside can fit slightly looser.