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What is MP4?

MP4 is a container format, not a codec: it wraps video (usually H.264), audio (usually AAC) and metadata in one file, and it is the closest thing to a universally playable video file.

MP4 at a glance

MIME type
video/mp4
File extensions
.mp4, .m4v
Specification
ISO/IEC 14496-14, based on the ISO base media file format
Compression
Depends on the codec inside. H.264 is the safe default; H.265 and AV1 are smaller but less widely supported.
Transparency
Not in any commonly supported codec profile.
Animation
Not applicable — it is video.
Colour depth
8-bit in the common H.264 profiles; 10-bit and HDR in H.265 and AV1.
Metadata
Title, creation date, rotation matrix, chapters and per-track language tags.
Support
Plays on every browser, phone, television and editor in ordinary use, provided the codec inside is H.264 with AAC audio.

Strengths and weaknesses

  • The one video file you can send to anyone without asking what they use.
  • Hardware decoding on essentially every device, so playback costs little battery.
  • Supports streaming with a single flag that moves the index to the start of the file.
  • H.264 is licensed technology, which is why open-source browsers historically resisted it.
  • Compression is behind newer codecs — AV1 and H.265 produce smaller files at the same quality.
  • The extension tells you nothing about what is inside; an MP4 can hold codecs a given device cannot play.

Container versus codec

Almost every "this MP4 will not play" problem is a codec problem, not a container problem. The file is a valid MP4; the video track inside it is H.265 or AV1 and the player only speaks H.264.

Re-encoding to H.264 with AAC audio solves it. That is what our video converter targets by default, precisely because the point of an MP4 is that it plays anywhere.

Faststart, and why a video will not begin until it is fully downloaded

MP4 keeps an index of the file — the moov atom — which many encoders write at the end. A browser streaming such a file has to fetch the whole thing before it can show the first frame.

Moving that index to the front costs nothing and makes the video start immediately. Our encoder does it on every output.

Working with MP4 files

Convert to MP4

Convert or edit a MP4 file

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