Ring Size Converter: Korean 호, US, UK, EU, Japan
Every ring size system is a different name for one measurement: the inner circumference of the band. Once you know how each system defines itself, the conversions stop being a lookup table you have to trust and become arithmetic you can check.
A ring has one dimension that matters — the distance around the inside of the band. Korea and Japan number that distance one way, the US another, the UK with letters, and the EU by just stating it in millimetres. Nothing else differs. If you can get the circumference of your finger in millimetres, every system follows from it.
This converter works that way round: whatever you enter is turned into a circumference first, and every other system is derived from that number. That is why it can tell you when a size is exact and when it is only the nearest one that country actually sells.
How each system is defined
Korean 호 and Japanese 号 are the same scale, and it is defined on the inner diameter: size 1 is 13.0mm across, and each step up adds exactly one third of a millimetre. So 13호 is 13 + 12/3 = 17.0mm in diameter, or 53.4mm around. Size 27 works out to 21.7mm and 68.1mm. Because the definition is a formula rather than a chart, there is no gap in it — every whole number from 1 to 27 is a real size.
US sizes are a straight line in circumference instead: roughly 36.5mm at size 0, plus 2.55mm for every full size. That makes a US half size worth about 1.28mm of circumference.
EU sizes under ISO 8653 are the least mysterious of all — the number is the circumference in millimetres. An EU 53 ring is 53mm around. UK letters are the one system with no closed form, so they stay a lookup, though they climb at the same rate as the US scale, about 1.28mm per letter.
Verified conversions across the common range
Diameter and circumference are the 호 definition; the US and UK columns are the nearest size those systems sell, with ≈ marking the ones that are not an exact match.
| 호 / 号 | US | UK | EU | Diameter | Circumference |
|---|---|---|---|---|---|
| 9 | 5 | J½ | 49 | 15.7mm | 49.2mm |
| 11 | ≈6 | ≈L½ | 51.5 | 16.3mm | 51.3mm |
| 13 | ≈6.5 | ≈M½ | 53.5 | 17.0mm | 53.4mm |
| 15 | 7.5 | O½ | 55.5 | 17.7mm | 55.5mm |
| 17 | ≈8 | ≈P½ | 57.5 | 18.3mm | 57.6mm |
| 19 | 9 | R½ | 59.5 | 19.0mm | 59.7mm |
| 21 | ≈10 | ≈T½ | 62 | 19.7mm | 61.8mm |
Why some conversions can only be approximate
One 호 is 1.05mm of circumference. One US half size is 1.28mm. The scales climb at different rates, so most 호 simply have no US equivalent — one US half size is worth about 1.22호, and a full US size about 2.44호.
The visible consequence is that neighbouring 호 sometimes round to the same US size. 11호 and 12호 both land on US 6; 16호 and 17호 both land on US 8; 22호 and 23호 both land on US 10.5. These are not duplicate rows or errors in the chart — they are what happens when a finer scale is projected onto a coarser one.
It also means converting in a circle can move you. Take US 6, convert to 호, convert back, and you may not return to US 6, because the 호 you landed on sits closer to a different half size. Whenever the exact fit matters, carry the millimetre figure rather than the size number.
Measure the circumference, not the diameter
If you have a ring that already fits, it is tempting to lay it on a ruler and read the inner diameter. That method is roughly three times less forgiving than measuring around your finger, and the reason is π.
Circumference is diameter multiplied by 3.14159, so any error in a diameter reading is multiplied by 3.14159 too. Misreading the diameter by half a millimetre — easy to do against a ruler, on a curved edge, with the band's thickness in the way — shifts the circumference by 1.57mm, which is a size and a half.
Measuring around the finger with a paper strip has no such amplification. A one-millimetre error in the strip is a one-millimetre error in the circumference, which is just under one 호. If you must measure a ring rather than a finger, measure it twice from different angles and take the larger reading.
Getting a reliable number
- •Cut a strip of paper about 10mm wide, wrap it snugly around the finger, and mark where it overlaps.
- •Measure the marked length in millimetres — that is your inner circumference. Enter it directly.
- •Measure at the end of the day. Fingers are at their largest then, and a ring that only fits in the morning is a ring you stop wearing.
- •If the knuckle is noticeably wider than the base, size to the knuckle — the ring has to pass over it.
- •Wide bands sit tighter than thin ones at the same size. Above about 6mm of band width, go up by half to a full size.
Extended FAQ
Do I need to convert between Korean 호 and Japanese 号?
No. They are one scale, defined identically. A Korean 13호 is a Japanese 13号. Charts that map 5호 to 7号 or 21호 to 22号 are in error — you can spot them because the offset jumps around (+1, then +2, then +1) instead of staying constant, which is what writing 호 numbers alongside US rows produces.
Why does the result sometimes show ≈ in front of a size?
Because no exact equivalent exists in that system. The ≈ means the tool has given you the nearest size that country actually sells, and the fit will be slightly off. Sizes without the marker match to within 0.2mm of circumference.
Are there half sizes in the 호 system?
Not conventionally, but you rarely need them. A 호 step is 1.05mm of circumference, already finer than a US half size at 1.28mm, so the 호 scale gives you more choices across the same range, not fewer.
My jeweller's chart disagrees slightly with this one. Who is right?
Probably both. The 호 definition fixes the diameter to a third of a millimetre, but individual shops round to their own mandrel sizes and can land about 0.2mm either side. That is well inside the amount your finger changes over a day. Differences larger than about half a millimetre suggest the chart, not the rounding.
If I fall between two sizes, which should I choose?
The larger one. A ring that is slightly big can still be worn and can usually be resized down; one that is too small cannot go on at all. The exception is a narrow band, which spins when loose — for a half-size gap on a thin ring, try it on in person.
Are my measurements stored anywhere?
No. The conversion is arithmetic that runs in your browser, and nothing is sent to a server.