What is Aspect ratio?

Aspect ratio is the relationship between the width and height of an image or video dimensions. Aspect ratio is expressed by 2 numbers separated by a semi-colon.

The most popular aspect ratio would be 16:9 which means a width of 16 units with a height of 9 units (widescreen). 16:9 aspect ratio in video is the common 3840×2160 (4k), 1920×1080 and 1280×720.

An aspect ratio of 1:1 is a perfect square because the width has the same units as length. The need for social media video that gets viewed on a vertical screen means an aspect ratio of 1:3 would be ideal and fit the screen, whilst 16:9 would leave the mass of black space below and above the video.

21:9 on a 16:9 screen
21:9 on a 16:9 screen (note black bars)

Aspect ratio is more relational to the media holding its shape on different screen sizes which sets it apart from plain dimensions. You may have a 1920x1080p (16:9) monitor yet a 21:9 video has black bars on this screen. This is 21:9 media is ultra-widescreen and cannot fill a 16:9 screen fully.

video aspect ratios

The wiki gif above shows the differences with 4:3, 16:9 and 21:9 on those screen ratio.

21:9 numerically seems bigger than 4:3 but this is aspect ratios and the media cannot fill out a different aspect ratio without cropping.

21:9 on a 4:3 screen
21:9 on a 4:3 screen

Social media images such as Instagram and Twitter suit aspect ratio of either 1:1 for a square or 9:16 to fill the screen vertically.