Measure once, convert everywhere
Every shoe numbering system is trying to describe the same thing — how long your foot is — but only one of them says so directly. Japanese sizes are the foot length in centimetres, so a shoe marked 26.0 is built for a 26.0 cm foot. US, UK and EU numbers are conventions layered on top of that, which is why they never line up neatly.
So the reliable order is: measure your foot in centimetres, then convert. Stand on paper with your heel against a wall, mark the tip of your longest toe, and measure from the wall to the mark. Do both feet and use the longer one — a 5 mm difference between feet is common and nobody notices until a shoe pinches. Measure in the evening, because feet swell over the day.
How the systems relate
- Japan — foot length in centimetres, in 0.5 cm steps. The most literal system there is.
- US men's — roughly the centimetre length minus 18. A 26.0 cm foot is about US 8, 27.0 cm is US 9.
- US women's — about 1.5 higher than men's for the same foot. That 26.0 cm foot is US 9.5 in women's numbering.
- UK — about one size below US men's, and about 2.5 below US women's. US 9 men's is UK 8; US 8 women's is UK 5.5.
- EU — roughly the centimetre length plus 15, rounded to the half size. A 26.0 cm foot is about EU 41.
The men's-to-women's gap is the one that catches people out. A shoe listed as women's US 9.5 and one listed as men's US 8 are close to the same physical shoe, which matters when a style you want is only stocked in the other department.
Why the same marked size fits differently
Internal length varies by 2 to 5 mm between brands at the same marked size, because the marking is a convention rather than a measurement. That is more variation than a half size in some ranges, which is why "I'm a US 9" is less useful information than it sounds.
The fix is to work in millimetres. Most brands publish their own size guide with a centimetre or insole-length column — match your measured foot against that instead of trusting the number on the box. Japanese and Korean brands make this easy since their sizing is already in centimetres and millimetres.
Women's clothing across US, UK, EU and Japan
Clothing has no equivalent of foot length to anchor it, so conversions are looser still. The rough alignment is that US and UK differ by four (US 4 is UK 8), and EU numbers run about 32 higher than US (US 4 is EU 36).
Japan uses 号 (gō), which does have a measurement behind it: the JIS standard sets 7号 at an 80 cm bust, 9号 at 83 cm, 11号 at 86 cm, adding about 3 cm every two sizes. 9号 is the Japanese standard size, chosen because a national survey put the average adult women's bust near 83 cm.
Be aware that American cuts generally run larger than Japanese and European ones at the same nominal conversion. If a Japanese listing gives a bust measurement, that number is worth more than the size label.
What this converter assumes
- Japan (shoes) = foot length in cm · US men's ≈ cm − 18 · US women's = US men's + 1.5
- UK ≈ US men's − 1, or US women's − 2.5 · EU ≈ cm + 15, rounded to the half size
- China (shoes) = (mm − 50) ÷ 5
- Japanese clothing 号 = JIS L 4005 bust sizing (7号 = 80 cm, +3 cm per 2 sizes)
- All cross-country clothing mappings are approximate — brand variation exceeds system variation