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

Hat sizes are determined by measuring the circumference of a person's head about 1/2 inch (1.3 cm) above the ears. Inches or centimeters may be used depending on the manufacturer. Felt hats can be stretched for a custom fit. Some hats, like hard hats and baseball caps, are adjustable. Cheaper hats come in standard sizes, such as small, medium, large.
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.