US: S to United Kingdom Size 6 3/4
Cross-market women’s hat conversion mapping S specifications into 6 3/4.

Analyzing Hat Size Equivalence Across Markets
US: S: Features a head circumference of 54.5 (21.5 ) measured just above the brow and ears.
United Kingdom Size 6 3/4: Features a head circumference of 55.5 (21.9 ) measured just above the brow and ears.