Finding that "perfect" fit can be a real challenge! It's our hope the size chart we created functions as a helpful tool when making your buying decision. As parents ourselves, we realize the world of baby sizes is definitely not one size fits all.
If your little one has some seriously adorable chunky feet and legs, it's probably best to size up. If they are on the long and narrow, you might find they get more longevity out of the smaller sizes. We've had petite 2.5 year old's that still fit in a 6-12 and 12-24 and even five year old's (with narrow size 10/11 shoes) that fit in our 2T-3T.
On the flip side, we've also had 5 month old's fit in our 12-24 and 2.5 year old's (with size 8 shoe) fit perfectly in our 2T-3T. Ultimately, we say use your amazing, superpower parental and caregiver intuition, it's usually right!