Oh lord every designer is different. It drives me crazy. I’m a US 9.5 very consistently, but between a size 39.5-41 EU or IT depending on the designer. I make sure I know what my size is for that brand before I buy. What brands are you looking at? More on reddit.com

i dont know. i havent found an authoritative source really, either. even the wikipedia page mentions a conflict between its own chart (the organization of which i love, but which is definitely wrong about at the very least US womens' sizes) and zappo's guide. More on reddit.com

Because there isn't an exact conversion between the scales that end up with whole or even half sizes, and different brands fit differently anyway, it's very much a guideline than a rule. I've more often seen 36s sold as 6s in Canada, which is a whole other world of confusion, since a 36 is much closer to a 5.5 in my experience. Also, European shoe lasts tend to run a little wider than NA ones, so an entirely different sizing factor complicates things as well. Shoes that tend to run wide are often treated as "sister sizes" are in bras, that is, if you're a 6W then you can probably wear a 6.5 in medium width. (This saves my ass big time, since I'm a about a 5WW, so I can wear most sixes comfortably, and 36s are very close, since that's more like a 5.5W.) Shoe sizing is exactly like clothing sizing, there's no true standard and every manufacturer is slightly different. Sizing even changes depending on shoe design and style. It doesn't really matter where you sit in the size range, this can be difficult for everyone. And this doesn't take into account height of toe box and arch, which are at least as important for fit as length and width. This doesn't help wrt online shopping, of course, but the best advice is to not get married to a number when it comes to shopping for shoes, same as for clothes. More on reddit.com

It’s wrong. At least the European and American ones don’t match up. More on reddit.com