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

Analyzing Hat Size Equivalence Across Markets
US: XS: Features a head circumference of 53 (20.9 ) 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.