Ring Size Converter (Korea/Japan 호, US, EU, inner diameter)

Understanding the ring size converter — concept and principle

This converter translates a ring size between the Korean/Japanese 호 system, US/Canada sizes, EU (ISO 8653) sizes, approximate UK letters and the inner diameter in millimetres. It helps when you buy a ring abroad or online, or when you know your size in one system and a shop uses another.

How it is calculated

Every size is first turned into the ring's inner circumference C in millimetres, and each other system is then derived from it:

• Korean/Japanese 호: C ≈ 40 + size, so size ≈ C − 40.
• US/Canada: inner diameter d = 11.63 mm + 0.8128 mm × size, so size = (d − 11.63) ÷ 0.8128, rounded to the nearest half size.
• EU / ISO 8653: the size number is simply C in mm.
• UK (approx.): letters start near A ≈ 37.8 mm and step about 1.25 mm per letter.
• Inner diameter: d = C ÷ π.

The principle

A ring fits by its inside measurement, and circumference and diameter are linked by C = π × d. ISO 8653 uses the circumference directly, which is why a paper strip around your finger gives the EU size. The US scale is linear in diameter: size 0 is 11.63 mm and each full size adds 0.032 inch (0.8128 mm). The Korean and Japanese scales add roughly 1 mm of circumference per size, so they sit about 40 mm below the EU number. These are approximations of trade charts, not one single legal standard.

Worked example

Default input: Korean/Japanese size 11호.

1. Circumference: C = 40 + 11 = 51 mm.
2. Inner diameter: d = 51 ÷ π = 16.23 mm.
3. US size: (16.23 − 11.63) ÷ 0.8128 = 5.66, rounded to the nearest half = 5.5.
4. EU size: 51. UK: (51 − 37.8) ÷ 1.25 = 10.56 → 11 steps after A = L.
5. Result: inner circumference 51 mm, diameter 16.23 mm, US 5.5, EU 51, UK L.

How to read the result

A difference of one Korean size is only about 0.3 mm in diameter, yet it is easy to feel, so a half size matters. Larger circumference means a bigger ring in every system. If the US value lies between two half sizes (5.66 here), try both, and choose the larger for wide bands.

Common mistakes

• Measuring a cold finger in the morning; fingers swell later in the day and in heat.
• Using a stretchy string, which gives a smaller number than a paper strip.
• Measuring a ring's outside diameter instead of the inside.
• Ignoring the knuckle: the ring must pass over it, so measure there if it is larger.
• Assuming wide bands fit like thin ones; a wide band often feels tighter at the same size.

FAQ

Frequently asked questions

What US size is Korean 11호?

About US 5.5 (between 5.5 and 6), with an inner circumference near 51 mm.

How do I measure my ring size at home?

Wrap a thin strip of paper around the base of the finger, mark the overlap and measure the length in mm. That length is your EU size; subtract about 40 for the Korean size.

Can I use a ring I already own?

Yes. Measure its inner diameter across the middle in mm and multiply by π to get the circumference.

Sources: ISO 8653:2016, Jewellery and precious metals — Rings — Definition, measurement and designation of ring size