I think a 4 is the average for that age. Some shoes go by months as well, such as 12 to 18 months, which is what my 10 month old son is wearing in his Pedipeds, although there's quite a bit of growing room.
we got DS fitted at stride rite. He is apparently a 4.5W. When I put shoes on him, it seemed a 3.5 was a better fit (toes not quite to the front, but close), but stride rite assured me the 4.5W was what he needed, there is still a fair amount of room for his feet to grow, but not overly sized.
My twins both wear size 5 although my grandmother(who worked for a podiatrist for several years) says to make sure that the shoe covers the ankle if they are going to be walking in them and they have half an inch of growing room.
My son was more 12-18 months size for Pedipeds when he was 9-10 months. And then he was more size 4 or 4.5W. We got him his first pair of shoes for his 1st birthday. When they measured him, I wish we would have gotten the shoes in a 1/2 size up, so he could have worn them longer. It drives you crazy when you pay $50 for Stride Rite shoes, and he did not wear them very long. And he kept trying to take them off in the grocery store and throw them off.
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Re: What size shoe does 10-12mo old boy wear?
https://www.trimfootco.com/shoe-size-guide.html
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