How the four systems relate
US men's and UK sizes run closest together — UK is typically half a size below US men's. US women's sizes sit about 1.5 above US men's for the same foot (a men's 8 is a women's 9.5). EU sizing uses a different unit entirely, Paris points of 6.67 mm, which is why EU sizes climb in a different rhythm and land on half sizes like 41.5. None of the systems measure the foot directly — they measure the shoe's last, which is where brand variation creeps in.
Why brands disagree with the chart
Conversion charts are conventions, not regulations. Each manufacturer decides its own last dimensions, so a Nike 42 and an adidas 42 can fit differently, and some brands deliberately run a half size off the standard chart. Width matters too — a US D width (men's standard) differs from a 2E or a women's B — and most conversion charts, including this one, only handle length.
The reliable method: measure in centimeters
Trace your foot in the evening (feet swell during the day), measure heel to longest toe in cm, and add about 1 cm of room. Then check the brand's own size chart, which almost always lists cm or 'foot length'. Japanese sizing and many running-shoe charts use cm directly — it's the closest thing shoe sizing has to a universal standard.
How to use this converter
Choose the system you already know — US men's, US women's, UK, or EU — and your size, and the tool shows the equivalent in all four with your row highlighted in the full chart. It's built for the common cases: ordering from a site that lists only EU sizes, or converting a men's size to women's and back.
A worked example
You wear a US men's 9. The chart lines that up with roughly a UK 8.5, an EU 42.5, and a US women's 10.5. So if a European brand lists only EU sizes, reach for the 42.5; if you're eyeing a women's colorway, try the 10.5. Because brands cut on their own lasts, treat these as a starting point and confirm against the brand's own centimeter chart whenever one is available.