US: XS to Europe Size 54
Cross-market men’s hat and headwear conversion mapping XS specifications into 54.

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 54: Features a head circumference of 54.5 (21.5 ), measured above the ears where the hat sits comfortably.