TVs generally come in rectangular shape. So, you need to calculate the diagonal of a rectangle using the settings of your TV screen. Diagonal in any polyhedron represents any segment that connects two vertices that are not adjacent.
In order to accurately determine the diagonal of your TV, measure its side. Remember that the screen starts not from the edge. On the edge of the screen, a part of the body, the size of which should be measured on the inner side. You need a diagonal that is screen and not the whole panel of the TV. That is the visible part.
Then remember the Pythagorean theorem and using this theorem calculate the desired diagonal. The formula of the Pythagorean theorem is the following: d = √(a2 + b2). d in this formula is the diagonal, a and b are the sides of the rectangle. Substitute the dimensions of the sides of the TV and get the diagonal size.
The disadvantage of this measurement is the diagonal of a TV is to measure the length of the sides, with a millimeter precision, you hardly will succeed. To do this with a ruler is difficult. Take some thread or string, stretch it along one of the sides of the TV, and then measure with a ruler. Do the same with the second side. Such a measurement method will be a little more accurate.
But if you decided to use a rope or thread, you can try to measure the diagonal without using the Pythagorean theorem. To do this, tighten the thread ends between nonadjacent vertices of a TV screen, and then measure the resulting length with a ruler.
To navigate the international dimensions, translate the results obtained into inches. 1 inch is equal to 2.4 cm, So you get the size of the diagonal of your TV set according to international standards.
There is another option that will probably be the easiest. Type in any search engine the model of your TV and find all the necessary technical characteristics.