Japanese ring size to US ring size
Pick the Japanese size number to see its US and EU equivalents.
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How to use it
- Enter japanese size.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Diameter = 13 + (Japanese size − 1) ÷ 3 mm, circumference = π × diameter. US size = (C ÷ (π × 25.4) − 0.458) ÷ 0.032.
Example
Inputs: Japanese size = 13. Result: US size: 6.5; EU size (ISO): 53; Inner diameter: 17 mm.
Limits and caveats
- Uses the JCS scale found in most Japanese shops.
- Sizes are rounded to the nearest step of each system, and shop mandrels can differ slightly.
Questions about this tool
What is Japanese size 9 in US sizes?
Its diameter is 15.67 mm, which is US size 5.
What is Japanese size 13?
A 17 mm diameter and 53.4 mm circumference, about US 6.5 and EU 53.
What is the difference between JCS and JIS?
JIS writes the circumference in mm directly, roughly the JCS number plus 40.