If you, too, believe the school teacher, the diagonal of a rectangle is the length between the opposite corners, passing through the center of the figure. Measure this segment of a rectangular screen of your monitor or TV. The tool can serve as a centimeter, measuring ruler, meter, or other available tool, which can be used without fear to damage the surface of the tube or matrix.
And if the school you still remember the Pythagorean theorem, it is possible to do even trickier to measure is the diagonal length and width, and the desired value to compute the root from the sum of the squares of the resulting values. Sometimes using this method is preferable as it guarantees the safety of the surface of a CRT or matrix.
Obtained in the previous steps, the number must be in inches. Usually in these units is indicated by the size of the screen, so most likely, you need it in those units. Every inch contains approximately 2.54 cm, so divide by this value, measured in centimeters the length of the diagonal.
Measure is meaningful only in cases where simpler methods are unavailable. For example, almost always on the body of the device with the screen you can find marking in which one of the numbers indicates the size diagonally. For example, if the designated monitor SyncMaster 2232BW, the first two digits in the numerical designation refer to the diagonal size in inches. If it is necessary to know the diagonal in inches, perform a mathematical operation, the inverse applied in the previous step - 22, multiply by 2.54.
Diagonal of the monitor screen by the numbers in its name, you can define not even examining the case. If the computer is turned on and the operating system is configured correctly, the full name along with the numbers can be seen by going to the settings screen. In Windows 7 and Vista, simply click on the desktop, right-click and select "screen Resolution". The full name of the monitor can be found in the "Screen" of the window.