US: M to Europe Size 54-55
Cross-market adult earmuffs and headbands apparel conversion mapping M specifications into 54-55.

Analyzing Size Equivalence Across Markets
US: M: Corresponds to a head circumference baseline of 57 (22.4 ) with a Standard / Regular fit type.
Europe Size 54-55: Corresponds to a head circumference baseline of 55 (21.7 ) with a Standard / Regular fit type.