US: 15.5–18 to United Kingdom Size 3.5–5
Cross-market men’s sock conversion mapping 15.5–18 specifications into 3.5–5.

Analyzing Sock Size Equivalence Across Markets
US: 15.5–18: Corresponds to a US shoe size range of 15.5–18, foot length of 32.5–35 (12.8–13.8 ), and fit classification Max Volume / Bariatric & Therapeutic.
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.