US: 11–13 to United Kingdom Size 3.5–5
Cross-market men’s sock conversion mapping 11–13 specifications into 3.5–5.

Analyzing Sock Size Equivalence Across Markets
US: 11–13: Corresponds to a US shoe size range of 11–13, foot length of 28.5–30 (11.2–11.8 ), and fit classification Relaxed / Comfort Stretch.
United Kingdom Size 3.5–5: Corresponds to a US shoe size range of 4–5.5, foot length of 22–23.5 (8.7–9.3 ), and fit classification Snug / Compact Fit.