To choose your hat size, measure your head circumference about 1 cm above the eyebrows and ears, and the bump at the back of the skull.
Be careful not to over-tighten, you want to have a bit of free space in your hat.
Women have a head circumference generally between 55 and 59 cm while men most often present a head circumference between 57 and 61cm. However, depending on your hairstyle, you may differ from the average. However, if you get a significantly high or ridiculously small result, it is likely that you have made a mistake.