Generally, a felt hat should fit snuggly, but not be so tight that you feel it compressing your head (unless you like that sort of thing). And avoid the fallacy of starting with an undersized hat thinking that it will loosen up with use — in fact, some felt hats will become a little tighter over time as they continue to self-felt with use and exposure to rain.
To achieve a proper fit, you should provide the milliner with a measurement of the circumference of your head, and sometimes the height, as follows:
1. Head Width: Use a flexible measuring tape or piece of string to measure the circumference of your head about 1 inch (2.5 cm) above your eyebrows. If you plan to wear the hat over your ears, hold the tape over the middle of your ears when you make the measurement. For a higher fitting hat, make the measurement just above the ears.For reference, most medium to large-sized hats that cover the ears have a circumference of approximately 58 cm (23 in.).
2. Head Height: Most hat designs will fit a wide range of head shapes, and the milliner will not need to know your head height; however, it is important for some designs and some heads. To measure head height, use a flexible tape to measure the distance over the top of your head from the top of one ear to the top of the other ear. You only need to make a small accommodation for the thickness of your hair, unless you have a very robust coif up there.