Phantom Aquatics 3mm Neoprene Fin Socks Size Chart for Men's: XS 5 | SM 6-7 | MD 8-9 | LG 10-11 | XL 12-13 | Size Chart for Women's: XS 6-7 | SM 8-9 | MD 10+ 3 millimeter neoprene keeps feet warm | Nylon II neoprene is very durable and stretchy Blind stitched seams lay flat on skin, Seams glued to minimize water entry Honeycomb silkscreen bottom provides traction Makes a great liner for hunting or hiking boots!