An aspect ratio describes the relationship of an object's width to its height. It is commonly used in computing to describe the proportions of a rectangular screen.
Aspect ratios are written as mathematical expressions, using the following format:
For instance, a monitor that is 20 inches wide by 15 inches tall has an aspect ratio of 20:15. After reducing the fraction (dividing each number by the lowest common denominator – in this case, 5), the aspect ratio is 4:3, or "four by three." 4:3 was the aspect ratio used by standard definition (SD) televisions, before HDTV.
A square screen or image has an aspect ratio of 1:1. A screen that is twice as tall as it is wide has an aspect ratio of 1:2. If a screen 50% wider than it is tall, its aspect ratio is 3:2. Both HDTVs and 4K televisions have aspect ratios of 16:9 ("sixteen by nine"), meaning they are almost twice as wide as they are tall. An HDTV, for example, has a resolution of 1920x1080 pixels. The 16:9 aspect ratio of HD resolution can be verified using the math below.
1920 ÷ 16 = 120. 120 x 9 = 1080.
Alternatively, 1920 ÷ 120 = 16. 1080 ÷ 120 = 9.
4K is simply twice the width and height of HD, or 3840x 2160. Therefore, 3840 ÷ 240 = 16. 2160 ÷ 240 = 9.
While most modern televisions have 16:9 aspect ratios, other types of screens may be longer or taller. For example, many tablets and computer monitors have 16x10 (or 8x5) aspect ratios, which means they are slightly taller relative to a standard HD display. Smartphones often have extra long screens when held sideways (a.k.a. landscape view). For example, the Samsung Galaxy S8 has an aspect ratio of 18.5:9 and the iPhone X has an aspect ratio of 19.5:9. When watching HD video on these devices, the video does not fit the full width of the screen. Instead, black bars are displayed on the sides because the aspect ratio of the video is not as wide as the aspect ratio of the screen.
NOTE: HD (16:9) is 25% wider than SD (4:3 or 12:9). That is why modern flatscreen displays appear wider than older CRT televisions.