Video resolution and aspect ratios
YouTube displays videos with different aspect ratios based on the platform and video format. The YouTube video player automatically adapts to the size of each individual video.
The YouTube app for Android mobile automatically adapts the player to the size of the video. Whether it's vertical, square or horizontal, the video will fill the screen. The vertical (portrait) video player matches the aspect ratio of the content – getting taller for square and vertical videos, and shorter for widescreen videos.
How vertical videos are displayed
Vertical videos will now be displayed in the video player without black bars on the sides. See example:
A viewer can tap the square button on the bottom right-hand corner of the video to enter into vertical full screen to view the entire video.