To measure your hat size, place a cloth/soft tape measure around the circumference of your head following your forehead and about ⅛ an inch above your ear (or wherever you would like your hat to sit).
Do not pull the measuring tape too tight but rather at a comfortable fit.
If you do not have an accessible tape measure, use a string instead and measure the desired length against a ruler.
Note that your hat will adjust even more to your head with each wear and become even more comfortable over time. Just like a new pair of shoes!