Sock sizes refer to the foot length they're knit to fit, expressed as a range.
Common sock sizes and shoe equivalents: 6–8 = kids US 12–4, 9–11 = US shoe 5–9, 10–13 = US shoe 9–12, 13–15 = US shoe 12–15.
Athletic socks are usually one size labeled 'L' or 'XL' that stretches across multiple shoe sizes.
Compression socks need exact fit by ankle and calf circumference, not shoe size. Always measure both.
Wool socks shrink. If buying merino or wool blend, size up half a size.