Determine the screen size of your computer monitor by measuring the screen's diagonal, or the distance between the opposite corners of your monitor. The diagonal is generally measured in inches.
The estimation of the screen size of a monitor does not take into account the display aspect ratio of the monitor. This implies that a 4:3 21-inch screen has a larger display area than a 16:9 21-inch screen.
A monitor's screen size is not the physical size of the unit. It is the amount of display that is available for graphics or text without obstruction from different features of the computer's design, such as its casing.