How this conversion works
A ring's Korean size number tracks its inner diameter in a straight line: size 1 corresponds to an inner diameter of 13.0 mm, and each size step up adds about 1/3 mm. From that diameter, the inner circumference follows from basic geometry (diameter × π), and the US size is a linear approximation built from published inner-diameter reference points.
- size — Korean ring size (호), starting at 1
- diameter — inner diameter (mm), the value that defines the Korean size
- circumference — inner circumference of the ring (mm) = diameter × π
More detail
Why diameter, not circumference, defines the Korean size
A ring's Korean size is defined by its inner diameter, not circumference — size 1 is set at exactly 13.0 mm, rising by 1/3 mm per size. The inner circumference shown here is simply derived from that diameter (multiply by π), which is useful when comparing against a string or paper-strip measurement wrapped around an existing ring.
Why the US conversion is approximate
US ring sizes are built on a different reference table of inner diameters (a US 7 sits at roughly 17.3 mm, for example), and the relationship to diameter is close to linear but not perfectly so across the full range. This calculator uses a linear fit of that relationship, which is accurate to well within half a size for common adult ring sizes.
Fitting tip. Finger size changes slightly with temperature and time of day (fingers swell in heat, shrink in cold) — measure in the evening at room temperature for a size that fits comfortably in most conditions, and size up slightly for wide bands, which fit tighter than thin ones.
Frequently asked questions
What US size is a Korean size 13 ring?
A Korean size 13 has an inner diameter of about 17.0 mm (13.0 + 12 × 1/3), or an inner circumference of about 53.4 mm, which converts to roughly US size 6.6 — enter 13 above to confirm.
What's the inner circumference of a Korean size 1 ring?
About 40.8 mm (inner diameter 13.0 mm), by definition of the sizing scale used here — the smallest common Korean ring size.
How much bigger is each Korean ring size?
About 1/3 mm of inner diameter per size step (roughly 1.05 mm of inner circumference) — so a size 14 ring is roughly 0.33 mm larger across (about 1 mm larger around) than a size 13, all else being equal.
Why might my ring not match the size on this calculator?
Korean ring sizing follows the Japanese inner-diameter numbering scale, and can differ by about half a size from a specific store's sizing rings. For an exact fit, always confirm with a physical ring sizer.