US: S/M to United Kingdom Size 6 7/8
Cross-market women’s hat conversion mapping S/M specifications into 6 7/8.

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 7/8: Features a head circumference of 56.5 (22.2 ) measured just above the brow and ears.