You can measure your head circumference by using a flexible tape measure. Measure one finger height above your ear and eyebrow, as the hat would sit level on the head. It is better to have the hat slightly loose than too tight. Most hats can be made to fit more "snug" to your head by placing a strip of material behind the inner sweatband of the hat.

Note-Below is a common sizing chart as a good guideline. However, sizes tend to vary slightly from vendor to vendor.