Using a measuring tape is the most popular and useful method known.
By wrapping the tape measure around your head, placing it approximately an inch above your ears (the point that your hat usually sits) you need to hold the measuring tape securely but not too tight. Make sure you have wrapped it completely around your and the point where the tape meets is where you will find your hat measurement. This measurement can now be used as a reference when buying a cap or hat.
These hats do not stretch!
Please pick a measurement that is slightly larger than your measurement. Example: If your head measures 22 inches, get a size 22.5