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Intermediate

MP4 Faststart for Web Playback

Faststart moves the MP4 moov atom to the beginning of the file so browsers can start playing immediately while the rest downloads.

What is Faststart?

MP4 stores metadata in a box called moov. If it is at the end of the file, players must download the whole file before playback. With-movflags +faststart, FFmpeg relocates the moov box to the beginning, enabling progressive playback.

Benefits
  • Instant playback start on the web
  • Better user experience on slow networks
  • No quality change; metadata relocation only
Notes
  • Works on MP4 files (container-level optimization)
  • Streams must be MP4-compatible for copy mode
  • Subtitles in MP4 are limited vs MKV

How to Enable Faststart

1

Convert MKV to MP4 (if needed)

If your source is MKV, convert to MP4 first. For compatible streams, use lossless remux; otherwise re-encode.

ffmpeg -i input.mkv -c copy output.mp4

If copy fails due to codec/container limits, re-encode video/audio.

2

Enable faststart during muxing

Add -movflags +faststart when writing MP4:

Copy streams (no quality loss):

ffmpeg -i input.mp4 -c copy -movflags +faststart output.mp4

Re-encode to H.264 + AAC with faststart:

ffmpeg -i input.mkv -c:v libx264 -crf 20 -preset medium -c:a aac -b:a 192k -movflags +faststart output.mp4
3

Verify faststart

Quick ways to verify:

  • Play the MP4 over HTTP and confirm immediate start
  • Use ffprobe -show_atoms -of default=nw=1:nk=1 output.mp4 and check moov appears early
  • Use any MP4 inspector to confirm moov precedes mdat
Tip

Faststart only rearranges metadata; file size and quality remain the same.

Technical Details

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