Manufacturers have identified TV screens according to their diagonal measurement for decades. Before companies began selling high-definition televisions, most TVs had the same aspect ratio, 4:3, so every screen with the same diagonal measurement had the same height and width. However, high-definition TVs now have a larger aspect ratio, usually 16:9.
You can measure your TV to find out the screen size, regardless whether it's high-definition or regular, and also find out its aspect ratio.
Lay a tape measure diagonally across the screen itself, not including the frame or cabinet. Start at the lower left or right corner and measure up to the opposite corner. The measurement rounded to the nearest inch is the number you'll usually see advertised as the screen size, such as a 20-inch screen.
Measure the width of the screen horizontally, not including the frame or cabinet, to figure the aspect ratio. Also measure the height of the screen vertically, not including the frame or cabinet.
Divide the width by the height. If the result is closer to 1.33, your screen has an aspect ratio of 4:3. If the result is closer to 1.78, your screen has an aspect ratio of 16:9.