Media quality on Youtube
Note, this info and assumptions are based on individual observations. It is not claimed to be comprehensive or accurate.
All popular video sites hold several media files with different qualities for each clip. Usually these files differ by resolution, there are original resolution and several lower resolutions to be used on smaller screens or to save bandwidth. So when determining best quality, all other video properties usually can be ignored.
Youtube goes further and in addition to resolution has several other properties. Combinations of these properties results in a big variety of possible qualities.
Each known property is described under own separate section below.
File formats and codecs
Youtube uses three file formats (or media containers): MP4, WebM and 3GP:
|Media container||Video codec||Audio codec|
|MP4||H.264 (MPEG-4 Part 10)||AAC|
|MP4 DASH||H.264 (MPEG-4 Part 10)||-|
|3GP||MPEG-4 Part 2||AAC|
Additional info about DASH files you can read on Adaptive streams page.
[Retrospective]. FLV was used by total majority of video sites in the past. Youtube had three qualities with it before adaptive streams appearance (,  180/240p, 360p, 480p FLV in 2012). After switch to adaptive streams in 2013 there were left only 180/240p . And it seems was finally removed from Youtube in Summer 2016 , .
Qualities list usually contains all common resolutions from 144p to original video resolution. Maximum possible resolution at this moment is 8k (4320p, 7680×4320 for 16:9 in ideal case) .
In general case every non-low quality is available within both MP4 and WebM formats. Depending on some reasons (like probably clip popularity) WebM adaptive streams may not be available yet for some clips .
|Video resolution||Media container|
|240p, 360p, 480p, 720p, 1080p, 1440p, 2160p, 4320p||MP4, WebM|
|144p||3GP, MP4, WebM|
Regular video files
Since DASH streams was added, most of regular video qualities (i.e. files containing both video and audio streams) was removed from the site. Now only few of them are available:
|Media container||Video resolution|
So maximum resolution of regular video file is 720p and only MP4 no matter of how high is original quality of the clip.
Youtube has support of high FPS (Frames per Second) videos. Most common frame rate is 60 FPS , but this depends on original video. For example it can be 48 FPS if original quality has this frame rate .
Non-high fps frame rate i.e. 'normal' frame rate in case of these clips is always high fps value divided by two. So it will be 30 fps for 60 fps original video, 24 fps for 48 fps original video etc.
Video with quality containing high fps is provided only within DASH streams. Its availability also depends on resolution and file format:
|Video resolution||Media container with high fps|
So resolutions up to 480p do not have high fps. 8k videos currently also don't have it. As for other resolutions, WebM has high fps for all of them but MP4 has it only for 720p and 1080p .
Not available anymore as of summer 2016.
[Retrospective]. Some clips on Youtube among with video having original image in different resolution also had few media files with 3D video generated from original file. Not to be confused with regular 3D clips which have all medias in 3D, these clips had only few variants of soft-made 3D.
Usually (or always) there were 360p WebM, 360p MP4 and 720p MP4 (, soft 3D marked with green circle; the same clip now: ). Stereo pair there always had horizontal layout and half-width image size.
Some clips may have VO medias with the same resolution, format and codec. The only difference is bitrate (in example: 2 vs 3 (15Mbps vs 18 Mbps), 4 vs 5 (22Mbps vs 24 Mbps). This breaks general auto comparing assumption that all videos with the same resolution have the same quality. However this is yet rarely seen situation. Video bitrate is yet not used at automatic comparing because higher bitrate does not always mean better quality: it is not true when comparing codecs from different generations i.e. h264 with 'next-gen' VP9 which claims the same quality with lower bitrate.
DASH audio streams can be freely combined with the video streams thus the audio format and quality also should be taken into account at selecting output quality. When downloading quality 720p and below, regular video+audio files also might be selected depending on desired output quality: if at comparing two qualities the video quality is equivalent, the program compares an audio quality.
Below is complete list of available audio qualities (including audio quality in regular video+audio files) in decreasing order of quality.
|Media container||Audio codec||Channels||Samplerate, Hz||Bitrate, kbps||Video resolution|
Best audio formats available on Youtube are 256kbps AAC in stereo and 384kbps AAC in 5.1. They are available only within DASH stream.
Note that both MP4 and WebM media containers have incompatible audio codecs among available audio DASH streams. AAC can be muxed only into MP4 and Vorbis, Opus - into WebM (this is controlled by 'Muxing exceptions' option). So for example in the clip below even if it has 256kbps AAC, the best quality is VP9+Opus: at video selection highest priority has video quality so the best video is WebM VP9 (2160p 60FPS) and it compatible only with Opus/Vorbis.
After adding support for HDR videos the champion (on 09-Nov-2016) is this clip (40 qualities).
This is not the first time when the clip has 40 qualities. This has happened before due to more qualities containing regular videos and adaptive audios.
Although it is not the first time, it still looks impressive:
Note, formats a1 - a5 are not separate files, they are audio streams from regular video files #19, #26, #27, #31, #35.
You may be also interested in ex-champions by the number of qualities.
Help Miscellaneous Media quality on Youtube