This guide is from the manufacturer. While shoe size helps you select the size of the socks, please remember that the youth size is also shorter than the adult sizes.
Please take the child's leg length into consideration when placing the order. For over the calf (or knee high styles)Youth size generally measure about 11" from heel to cuff; Adult Medium 13-14" and Adult Large 15-16" depending on style. This is the measurement hanging on the rack.
To allow for growth during the season you may want to select the next size up if the shoe size is close to the higher end of the sizing.