If your little piggies are going to market, these Shoe Size Conversion Tables can help. You can use them to convert from the shoe size used in one country or region to the shoesize used in another country or region. Note however, store and manufacturer practice varies widely. Having studied many of the shoe size tables published on the net, I would not trust any of them.
The best practice I can recommend, if the company allows this, is to place orders in terms of inches, millimeters, or centimeters. You can at least measure your foot and measure the shoe and determine if you received what you ordered. Otherwise, you might encounter a compounded error of both the manufacturer having a variation, and the store conversion table having a variation, which makes the shoe significantly different in size. I also notice that many tables and/or companies cite rules which only work for men, or women but not both. But often these tables don't indicate gender or are used for both.