When measuring your head, you should be using something that cannot be stretched, such as a flexible tape measure or a piece of string.
Wrap the tape around the widest possible circumference of your head - from the most prominent part of your forehead (for most people this will be about 1 to 2 fingers above their eyebrows) to the widest part of the back of your head.
Try to find the widest measurement around your head.
Remeasure 3 times and take the largest of them as your head circumference size.