How to establish thickness in yarns

Do you know how to establish or recognize thickness in yarns? In technical jargon the thickness of the yarns is called “Count”, often represented with the symbol “Nm.” which means Numeric Metric.
It is always based on a quantity of 1 kg of yarn.
The thickness of the yarn cannot be measured with the gauge, and therefore this ratio is used. Check out the video below by Alicja Kwartnik, it’s explained very well.
The video is in Italian but it can be understood very well anyway.
Video created by Alicja Kwartnik