Young Brunette Girl With A Blue Scarf Wearing A Hat On The Hill Above The Sea

Traditional hat size is worked out by adding the fore and aft and side to side measurements (in inches) then dividing by two. In the UK, an equivalent hat size is an eighth of an inch smaller than in the US.
Hat sizes in different types
Circumference in cm 34 43 47 48 49 50 51-52 53-54 55-56 57-58 59-60 61-62 63-64 65-66