I'm selling this hat because my baby quickly outgrew it, even though it was a gift set with a blanket. The inside of the hat measures about 38cm. It stretches, so I think it would fit comfortably with a head circumference of around 40cm. I still use the blanket and love it; it's KASHWERE, so it's incredibly warm♡ The texture is also wonderful, and I wish my baby could have used the hat more ( ; ; ) I used it a couple of times for a few minutes each. I will wash it at home before shipping. #baby #hat #KASHWERE