My left foot measures a perfect 12  M. My choices for these inserts were 11+, or 11-13. I chose the larger pair. The problem was the space from the heel to the arch. It was too long and couldn't be adjusted. The length was about 1 1/2 &#34; too long, which could have cut to fit if the arch area had fit. Hate to pay hundreds for a Rx fit but don't have a choice.