Calculating Your Own Hat Size.This chart is for your reference,the hat has one size, it is for you exchange "CM" to your countries' "Inch"

To calculate your metric (European) hat size a measurement of the head circumference should be taken in centimeters. This is best achieved by passing a cloth tape measure around the head. Alternatively a piece of string may be used and measured against a ruler or tape measure. It is important that this measurement is taken where a hat would normally rest. To take a measurement position the tape measure or string so that it rests just above the brow at the front of the head, touching the top of the ears at the side. Pull the tape so that is taut but not too tight and take a measurement.