US: 6–8 to United Kingdom Size 3.5–5
Cross-market men’s sock conversion mapping 6–8 specifications into 3.5–5.

Analyzing Sock Size Equivalence Across Markets
US: 6–8: Corresponds to a US shoe size range of 6–8, foot length of 24–26 (9.4–10.2 ), and fit classification Standard Athletic / Form Fit.
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.