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Glossary · format

What is Fragmented MP4?

Fragmented MP4 (fMP4) is an MP4 variant where the file is split into many short chunks ("fragments"), each containing its own header. This makes the file streamable in real time — a player can start decoding the first fragment before the full file has arrived. YouTube and most modern streaming services use fMP4.

Also called:fmp4 · fragmented mpeg · streaming mp4

A standard MP4 file has a single index ("moov") at the start (or end) describing where every frame lives. Players need that index before they can decode anything — which means a partial download is unplayable. Fragmented MP4 fixes this by interleaving small index pieces ("moof") with their data, so each fragment is independently parseable.

For streaming protocols (HLS, DASH), fMP4 is mandatory. The CDN serves a small fragment as soon as it's ready instead of buffering the whole file. For downloads, fMP4 is mostly invisible — most players handle it identically to standard MP4.

YouTube's download URLs serve fMP4. VidPickr's in-browser muxer detects fragmented inputs and assembles them into a single playable file with a unified index, so the saved file works in QuickTime, VLC, and every other player that handles standard MP4.

Common questions

Will a fragmented MP4 play in QuickTime?
Yes for fragments inside a properly written file. Direct playback of a single fragment may fail; full files with fragmented internals play normally everywhere.

Related terms

Mux (muxing)

Muxing is the process of combining separate audio and video streams into a single container file (MP4, MKV, WebM).

DASH (MPEG-DASH)

DASH (Dynamic Adaptive Streaming over HTTP) is an open streaming protocol.

HLS (HTTP Live Streaming)

HLS (HTTP Live Streaming) is a streaming protocol developed by Apple.

Container (file format)

A container is the file format that wraps one or more audio and video streams into a single file.

VidPickr is a free, browser-based YouTube downloader. Every term in this glossary either describes how YouTube delivers video or why your downloads behave the way they do. Try the downloader →