Calculating the screen size of a television set requires using the Pythagorean theorem for right triangles. The measurement used for describing a television is the diagonal length of the screen. The two most common screen formats, SDTV or HDTV, have aspect ratios of 4:3 and 16:9, respectively. Given this, SDTV screen measurements are 0.8 times the diagonal for the width and 0.6 for the height; HDTV screens are 0.87 times the diagonal wide and 0.49 times the diagonal for the height.

Therefore, a 65-inch television screen is either 52 inches by 39 inches for an SDTV or 56.55 inches by 31.85 inches for an HDTV. These measurements include just the screen size and do not describe the area beyond that for the set enclosure. For a television that large, adding a few inches per side is necessary to determine the total size of the set.