Use a cloth or soft tape measure to find the circumference of your head. Wrap the tape around your head where the hat will sit - typically just above your eyebrows and about ⅛ inch above your ears.
Make sure the tape sits comfortably - not too tight and not too loose. It should feel comfortable, similar to how you’d want your hat to fit.
No tape measure? No problem. Use a piece of string instead. Wrap it around your head in the same way, then lay the string flat and measure its length against a ruler.
If you're basing your size on a favorite hat, be sure to check that brand’s sizing guide. Hat sizing can vary slightly between brands, with some running a bit larger or smaller.
Keep in mind: Hats often mold slightly to the shape of your head with wear, becoming even more comfortable over time - just like a favorite pair of shoes.