If you're going to invest any amount of money in a hat, you want it to fit you properly. An ill-fitting hat can lead to a number of problems, including discomfort and even hair loss. Luckily, measuring yourself for a hat is a simple procedure and can be done in minutes at home.
Use a tape measure (preferably the flexible kind used in sewing) to measure all the way around your head, where the brim of a hat would go. This is generally right above your eyebrows.
Fit the tape measure around your head snugly, but not tightly. This will mimic the fit of your hat.
Remove the tape measure, and note the length measured. Convert this number into a hat size using a sizing chart for hat sizes