The screen size is in inches. One inch is equal to 2.54 cm of metric.
Because the screen is a rectangle, it is not necessarily a fixed ratio.
For example, it may be 4:3, or it may be 16:10 or other ratio. In this way, it is obviously irrational to use length and width as the basis. Similarly, if the area is used, the product of length and width must also be calculated. It is more complicated. In addition, as long as the two rectangles with the same diagonal length, regardless of their respective proportions, the area must be the same.
So the easiest way is to measure the diagonal of the screen. Since this is the most intuitive and easiest way to measure a panel in a panel factory, so it is used in the market.