To find your head measurement and hat size, measure around your head the way you wear your hat, usually tilted slightly upward to allow for the sloped down brim.
Most people wear their hats about a little finger or a quarter inch above the ear.
If you wear your hat differently, measure your hat size that way. Pull the measuring tape snugly but not tightly around your head.
Standard Small, Medium, Large, Extralarge head sizes are in bold. Metric head sizes are becoming the world standard and have supplanted the French head sizes even in France.
American hat sizes and British-Irish hat
sizes show no sign of being discontinued in their respective countries. The
well respected Borsalino Brand of Italy lists their hats in all three size
ranges-Metric, British and American.