When choosing a hat it is good to know your head size for a more custom fit. Here is a chart to determine the correct size.
Measuring your head
If you don't know your head size, here is an illustration to help. To measure for a hat, circle the crown of you head with a measuring tape. Double check yourself to make sure you have an accurate measurement. Lastly, match your measurement to the above sizing chart. Voila! You've got your size!