US: S/M to United Kingdom Size 6 5/8
Cross-market women’s hat conversion mapping S/M specifications into 6 5/8.

Analyzing Hat Size Equivalence Across Markets
US: S/M: Features a head circumference of 55.5 (21.9 ) measured just above the brow and ears.
United Kingdom Size 6 5/8: Features a head circumference of 54.5 (21.5 ) measured just above the brow and ears.