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

Analyzing Hat Size Equivalence Across Markets
US: M: Features a head circumference of 56.5 (22.2 ) measured just above the brow and ears.
United Kingdom Size 7: Features a head circumference of 57.5 (22.6 ) measured just above the brow and ears.