For rock climbing shoes: Choose soft shoes two to three sizes smaller than your street shoe size. We recommend you fit your trad shoes one to two sizes under your street shoe size. Remember to factor in stretch. Unlined slip-lasted shoes tend to stretch about one full size. Lined shoes stretch about a half size. Remember: a well-fit comfortable shoe will work better than an ill-fitting tighter shoe. For boots: Add one half size to your street shoe size. You should be able to slip one finger behind your heel when you are standing up with the boot unlaced and your toes touching the front. If the boot is too short, your toes will get smashed walking downhill. Too long and your heel will slide up and down and give you blisters.