US: XS to Europe Size 57
Cross-market men’s hat and headwear conversion mapping XS specifications into 57.

Analyzing Hat Size Equivalence Across Markets
US: XS: Features a head circumference of 53.5 (21.1 ), measured above the ears where the hat sits comfortably.
Europe Size 57: Features a head circumference of 57.5 (22.6 ), measured above the ears where the hat sits comfortably.