Shoe sizes are something of a mystery, but the customary British and American systems aren’t terribly complicated; they’re just based on old units of measure, like the barleycorn, which is one-third of an inch, and shifted so that the smallest size is 0 or 1.
For instance, in the American system, a men’s shoe is sized based on the length of the last, the foot-shaped template for the shoe, as measured in barleycorns, or thirds of an inch, starting at size 0 for an eight-inch foot:
US men’s size = 3 × last-length in inches – 24
American women’s shoe sizes are equal to men’s sizes plus 1.5:
US women’s size = 3 × last-length in inches – 221/2
American children’s shoe sizes are equal to men’s sizes plus 121/3:
US children’s size = 3 × last-length in inches – 112/3
British sizes, for both men and women, are equivalent to American men’s sizes minus one:
UK adults’ size = 3 × last-length in inches – 25
British children’s shoe sizes are equal to men’s sizes plus 13:
UK children’s size = 3 × last-length in inches – 12
In Europe they use Paris points, which are two-thirds of a centimeter, with no constant term:
European size = 3/2 × last-length in cm
I’m not sure what would be so bad about straight inches or centimeters.
What sells more shoes to French women, centimeters or Paris points?