The aspect ratio of a TV screen, computer, or device display is the ratio of its width to its height.
While the height and width might be naturally measured in inches or centimeters, for the purposes of aspect ratio, the actual screen resolution is used. For example, a device capable of displaying 1920 pixels by 1080 pixels is said to have an aspect ratio of 1920/1080, or 1.77. This is most often expressed as 16:9.
The so-called “standard definition” television typically has an aspect ratio of 4:3.
With the advent of high definition, the aspect ratio was changed to 16:9, meaning that an HD display is somewhat wider and more rectangular than then the older standard-definition format. All HD television formats – 720i, 720p, 1080i and 1080p – use the 16:9 aspect ratio. The 720 or 1080 corresponds to the height, so a 720 HD image is actually 1280×720, and a 1080 image is 1920×1080.
There is no standard aspect ratio for computers. Older common screen resolutions, like 640×480 and 1024×768, have aspect ratios of 4:3 (1.33), mimicking standard definition televisions, while resolutions such as 1920×1200 have a non-standard aspect ratio of 1.6.