Why shoe sizes do not line up
Each system counts in a different step from a different starting point. UK and US sizes go up in barleycorns of a third of an inch; EU sizes go up in Paris points of two-thirds of a centimetre; Japanese sizes and the CM column are simply the foot length in centimetres. Because the steps differ, an EU size never maps neatly onto a whole or half UK or US size, and every chart has to round.
The shoe size chart this tool uses
Every conversion goes through foot length, using the approximations given in Wikipedia's shoe size article: UK adult = 3 × foot length in inches − 23; US men's = 3 × foot length − 22, one size above UK; US women's = US men's + 1.5, the scale most retailers use; EU = 1.5 × foot length in cm + 2. For children, UK = 3 × foot length − 10 and US = 3 × foot length − 9.75. Sizes are rounded to the nearest half size. So EU 42 comes out as UK 8.5, US men's 9.5 and US women's 11, for a foot of about 26.7 cm.
Big kids' US sizes (1Y to 7Y) follow the men's scale, so for a foot longer than the kids chart, use the Adults tab and read the US men's column.
Brands vary, so measure
Manufacturers build their own lasts and their own charts, and the same foot can be half a size or more apart between brands — sometimes between two models from one brand. The most reliable figure you can bring to any chart is your foot length: stand on a sheet of paper with your heel against a wall, mark the longest toe, and measure in centimetres. Do both feet, late in the day, and use the longer one.