US: 3-6M to Europe Size 38-40
Cross-market children’s hats, beanies, caps, sun hats, and ear-flap hats apparel conversion mapping 3-6M specifications into 38-40.

Analyzing Size Equivalence Across Markets
US: 3-6M: Corresponds to a head circumference baseline of 42.5 (16.7 ) and age range 3–6 .
Europe Size 38-40: Corresponds to a head circumference baseline of 40 (15.7 ) and age range 0–3 .