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

Analyzing Hat Size Equivalence Across Markets
US: M/L: Features a head circumference of 57.5 (22.6 ) 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.