Measure your head before ordering. The order form will ask you to fill in your head measurement.
Use a flexible tape measure, such as a paper one, or a dressmakers tape measure. Don’t try to use a metal one.
Go around the fullest part of your head, just above the brow. Try it more than once, just to be sure. Don’t worry if you still have hair–it won’t make enough of a difference. Just pull the tape measure taught.
Tip: A typical adult head size is about 21″ to 22-1/2″. If you are getting a measurement outside of that, just double check it.