Japanese Ring Size Chart: Sizes, Inner Circumference, Inner Diameter, and the Formulas
A Japanese ring size is a standardized measure of the inside of a ring. But “size 9” or “size 11” doesn’t tell you much in millimeters. This article lays out sizes, inner circumference, and inner diameter in one chart, gives the formulas for converting between sizes and lengths, shows typical sizes for women and men, and explains how much difference one size makes on your finger.
What a Japanese ring size means: it’s set by the inner circumference
Ring sizes in Japan follow a JIS (Japanese Industrial Standards) notation based on the distance around the inside of the ring, the inner circumference. Size 1 is 41 mm around, and each size up adds 1 mm, so “size + 40” gives you the inner circumference in millimeters. Size 9 is 49 mm; size 13 is 53 mm.
The ring gauges used in shops (a bundle of loops, one per size) are made to these same circumferences. The outer size and thickness of a ring have nothing to do with its size number. It only describes the hole your finger goes through.
Chart of sizes, inner circumference, and inner diameter
Here are the commonly used sizes from 3 to 25. Inner diameters are calculated values to two decimal places; actual rings can differ by around 0.1 mm depending on how they’re made.
| Japanese size | Inner circumference | Inner diameter | Notes |
|---|---|---|---|
| 3 | 43 mm | 13.69 mm | Seen on little fingers and pinky rings |
| 5 | 45 mm | 14.32 mm | Slim women’s little and ring fingers |
| 7 | 47 mm | 14.96 mm | Slim women’s ring fingers |
| 8 | 48 mm | 15.28 mm | |
| 9 | 49 mm | 15.60 mm | A common size for women’s ring fingers |
| 10 | 50 mm | 15.92 mm | |
| 11 | 51 mm | 16.23 mm | Common for women’s ring and middle fingers |
| 12 | 52 mm | 16.55 mm | |
| 13 | 53 mm | 16.87 mm | Common for women’s middle and index fingers |
| 15 | 55 mm | 17.51 mm | Men’s little fingers, slimmer ring fingers |
| 17 | 57 mm | 18.14 mm | A common size for men’s ring fingers |
| 19 | 59 mm | 18.78 mm | Common for men’s ring and middle fingers |
| 21 | 61 mm | 19.42 mm | Men’s middle and index fingers |
| 23 | 63 mm | 20.05 mm | Larger men’s middle fingers and thumbs |
| 25 | 65 mm | 20.69 mm | Seen on thumbs (thumb rings) |
A full list of sizes 1 to 30, alongside US, UK, and EU sizes, is in the chart at the bottom of the Ring Size Measurer page. Tap a row to see that size’s inner circumference, inner diameter, and overseas equivalents together.
Formulas for converting between sizes and lengths
Even without a chart, these three formulas will get you a size from whatever number you have. A basic calculator is enough.
- Size = inner circumference (mm) − 40. Example: 51 mm → size 11
- Inner circumference (mm) = inner diameter (mm) × 3.14. Example: 16.2 mm → about 50.9 mm → size 11
- Inner diameter (mm) = (size + 40) ÷ 3.14. Example: size 13 → 53 ÷ 3.14 ≈ 16.9 mm
When the result lands between sizes, say 10.6, you’d normally go with the larger one. A ring that’s too tight won’t go on and hurts on the way over the knuckle, while one that’s slightly loose is easier to fix with something like a ring size adjuster. Some shops also offer half sizes such as 10.5.
Typical sizes for women and men
These are the guidelines jewelers commonly give for Japanese wearers. They only show the sizes that come up most often, and body build and finger shape will move them up or down.
| Finger | Women (typical) | Men (typical) |
|---|---|---|
| Ring finger | 9–11 | 17–19 |
| Middle finger | 11–13 | 19–21 |
| Index finger | 11–13 | 19–21 |
| Little finger | 5–7 | 13–15 |
| Thumb | 13–15 | 21–25 |
The same person’s fingers can differ by about 2 to 4 sizes, and the dominant hand is often slightly larger. Choosing from an average is easy to get wrong, so even when buying a gift, try to find a way to get a real measurement. The most reliable approach is to borrow a ring the person regularly wears and measure its inner diameter. Pick one worn on the same finger and hand the new ring is meant for.
How much difference does one size make?
One size is 1 mm of circumference, or about 2% of your finger’s girth. You may barely notice it on a thin ring, but on a wide band it clearly shows in how the ring feels.
- One size too small: it goes on but catches hard at the knuckle, and it’s difficult to remove when your fingers swell in the evening
- Just right: slight resistance at the knuckle, and once past it, the ring turns freely at the base of the finger
- One size too large: it feels like it might fly off when you shake your hand downward, and the ring rotates until the stone faces sideways
Finger size also changes by about 0.5 to 1 size between morning and evening and between winter and summer. For choosing with band width and knuckle size in mind, see What Size for a Wide Ring?
Measuring your own size
If you don’t know your size, measure the inner diameter of a ring that fits well, or wrap paper or string around your finger to measure the circumference. How to Measure Ring Size Without a Ring Sizer compares three methods you can do at home and where each tends to go wrong.
- 1
Calibrate the screen to real size
Open the Ring Size Measurer and hold a credit card or a 1 yen coin against the screen, adjusting until the frame on screen lines up exactly.
- 2
Place a ring, or read the string length
If you have a ring, match the on-screen circle to its inner edge. If not, wrap paper around your finger, mark it, and hold it against the on-screen ruler.
- 3
Check the size, circumference, and diameter
You’ll see the Japanese size, inner circumference, and inner diameter, along with US, UK, and EU sizes.
FAQ
- What is the inner diameter of a Japanese size 9 ring?
- Size 9 has an inner circumference of 49 mm and an inner diameter of about 15.6 mm. A Japanese size is the inner circumference in millimeters minus 40, and dividing the circumference by 3.14 gives the diameter.
- What’s the formula for calculating a ring size?
- Size = inner circumference (mm) − 40. If you only know the inner diameter, multiply it by 3.14 to get the circumference, then subtract 40. An inner diameter of 16.2 mm gives a circumference of about 50.9 mm, so roughly size 11.
- What is the average ring size for women?
- On the ring finger, sizes 9 to 11 are the most common range. Middle and index fingers tend to be around 11 to 13, and little fingers around 5 to 7. People vary a lot, so measuring is recommended.
- What is the average ring size for men?
- On the ring finger, sizes 17 to 19 are the most common range. Middle and index fingers are often around 19 to 21, varying with finger shape and build.
- What should I do if my size falls between sizes, like 10.5?
- Choose a half size if one is available. If not, the usual choice is the larger size (11). A tight ring may not get past the knuckle, while a slightly loose one is easier to fix with something like a ring size adjuster.
Tools in this article
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