From the Pattern:“Both the length and the circumference of your Sidewinders are set by the size you select. Since I’m shooting for a well-fitting sock, I’ve written the pattern for 12 sizes -- 4 different lengths each with 3 different widths. Please select the size based on you shoe size and/or actual foot measurements in inches. If measuring the circumference of your foot, please measure around the widest point -- typically around the ball of your foot.”