Smartwool Sock Size Chart

  U.S Men's U.S. Women's Euro U.K. Calf Circumference (for Compression Socks)
Small 3 - 5.5 4 - 6.5 34 - 37 2 - 4.5 11" - 14.5"
Medium 6 - 8.5 7 - 9.5 38 - 41 5 - 7.5 12.5" - 16.5"
Large 9 - 11.5 10 - 12.5 42 - 45 8 - 10.5 14.5" - 18"
X-Large 12 - 14.5 n/a 46 - 49 11 - 13.5 16" - 23"