We used the Pediped's sizing chart from their website as we moved from the Originals to the Grip-N-Go shoes. The sizing measurements change between these types of shoes. The Grip-N-Gos seem much larger on his foot than they should be, especially considering his Originals are very tight and the next size down. We've found that different shoes of the same numerical size are actually different sizes. It might be best to buy these in the store to ensure a good fit.