Determine where you want to wear your hat. Usually it is worn about an inch above the eyebrows and/or just above the ears. Have a friend hold the tape measure (cloth or plastic) at the point where you want the hat to rest on your head.  Insert the tip of your little finger between the tape and your head.  Record the measurement.  This should provide you with a comfortable fit.

If you have an old hat that fits comfortably, measure the inside circumference of the head opening of the hat.