1. Using a soft tailor's tape measure.
Take a tailor's tape measure and measure the circumference of your head along the line where you will wear the hat. The result will be your size. If the circumference is 57 cm, this means your size is 57.
2. Using a regular thick thread.
The thread should not stretch in length, otherwise the measurement will be incorrect. The thread must be wrapped around the head along the line, as indicated in the first method, and the resulting length of the thread should be measured with a regular ruler.