MyMediaTools

Convert MP4 to M4B — Free Online Tool

Convert MP4 video files to M4B audiobook format, extracting the AAC audio track and packaging it with chapter markers and bookmarking support. Ideal for turning video lectures, recorded interviews, or spoken-word MP4 content into a proper audiobook file that remembers your playback position.

FFmpeg Command

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Estimated output:

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How It Works

MP4 and M4B share the same MPEG-4 container structure, which makes this conversion highly efficient. The tool strips the video stream entirely and extracts or re-encodes the audio as AAC at 128k bitrate — if the source MP4 already contains an AAC audio track, the conversion is fast since minimal transcoding is needed. Any chapter metadata embedded in the MP4 is preserved in the M4B output, since both formats support the MPEG-4 chapter specification. Subtitle tracks and additional audio tracks present in the MP4 are not carried over, as M4B supports only a single audio stream and no subtitle tracks. The -movflags +faststart flag reorganizes the file's metadata to the front, which is a best practice for streaming-friendly playback in podcast apps and audiobook players.

What Each Flag Does

Flag What it does
ffmpeg Invokes the FFmpeg media processing tool. This is the same engine running in your browser via WebAssembly — the command shown is fully compatible with a local FFmpeg installation for processing files over 1GB.
-i input.mp4 Specifies the source MP4 file as input. FFmpeg reads the container and identifies all streams — typically a video track, one or more audio tracks, and optionally subtitles and chapters — before processing begins.
-c:a aac Sets the audio codec to AAC (Advanced Audio Coding), which is the required and universally compatible codec for M4B audiobook files. Any audio codec present in the source MP4 — whether it is already AAC, MP3, or Opus — will be encoded to AAC for the output.
-b:a 128k Sets the AAC audio bitrate to 128 kilobits per second. This is the standard bitrate for audiobook and podcast distribution — it provides clear, intelligible speech quality while keeping M4B file sizes compact, roughly 56MB per hour of audio.
-movflags +faststart Relocates the MP4/M4B metadata (the moov atom) to the beginning of the output file. This allows audiobook and podcast apps to begin playback immediately without downloading the entire file, and is a required best practice for M4B files distributed via podcast feeds or streaming.
output.m4b Specifies the output filename with the .m4b extension. The M4B extension is what signals to Apple Books, Overcast, and other audiobook players to treat the file as an audiobook with bookmarking and chapter navigation enabled, rather than a generic audio file.

Common Use Cases

  • Convert a recorded video lecture or online course module into an M4B audiobook so you can listen on your phone with automatic position bookmarking
  • Turn a video interview or documentary into an M4B podcast episode that supports chapter navigation in podcast apps like Apple Podcasts or Overcast
  • Extract the audio from an MP4 recording of a live audiobook reading or spoken-word performance and package it as a proper M4B with chapter markers
  • Convert a video sermon, conference talk, or keynote MP4 into an M4B file for distribution as a bookmarkable audio file to listeners who don't need the video
  • Prepare an MP4 narrated presentation for playback in audiobook apps on Apple devices, which natively support M4B but not MP4 for library-style listening
  • Strip video from a large MP4 recording to produce a much smaller M4B audio file for offline listening without consuming extra storage or bandwidth

Frequently Asked Questions

Yes — both MP4 and M4B use the same MPEG-4 container chapter format, so chapter metadata embedded in the source MP4 is preserved in the M4B output without any special handling. This is one of the key advantages of this conversion: apps like Apple Books, Overcast, and dedicated audiobook players can display and navigate those chapters directly. If your MP4 has no chapters, the M4B will simply play as a single continuous track.
The FFmpeg command re-encodes the audio to AAC at 128k bitrate regardless of the source codec. If your MP4 already contains an AAC audio track at 128k, this re-encode introduces a small quality loss from the generation of lossy compression. If the source is a higher-bitrate AAC track or uses a different codec like MP3 or Opus, you may notice a slight quality reduction. For spoken-word content like audiobooks and lectures, 128k AAC is generally transparent and indistinguishable from the source to most listeners.
The video stream is completely discarded. M4B is an audio-only format and has no support for video tracks, so the tool drops all video data and outputs only the audio. This is why M4B files are dramatically smaller than their source MP4s — a 1-hour lecture MP4 that might be 500MB can produce an M4B of under 60MB at 128k AAC.
By default, FFmpeg selects the first audio stream in the MP4, which is typically the default or primary language track. M4B supports only a single audio track, so all secondary audio streams are dropped. If you need to export a specific non-default audio track, you would need to modify the FFmpeg command locally by adding a stream selector such as -map 0:a:1 to choose the second audio track.
The audio bitrate is controlled by the -b:a flag in the command. The default is 128k, which is suitable for speech. To increase quality, replace 128k with 192k or 256k — useful if the source contains music or high-quality narration. To reduce file size for long audiobooks, you can lower it to 96k or 64k, which is still intelligible for speech. For example: ffmpeg -i input.mp4 -c:a aac -b:a 192k -movflags +faststart output.m4b.
The single-file command shown on this page processes one file at a time, but you can adapt it for batch processing in a terminal. On Linux or macOS, use: for f in *.mp4; do ffmpeg -i "$f" -c:a aac -b:a 128k -movflags +faststart "${f%.mp4}.m4b"; done. On Windows PowerShell, use: Get-ChildItem *.mp4 | ForEach-Object { ffmpeg -i $_.FullName -c:a aac -b:a 128k -movflags +faststart ($_.BaseName + '.m4b') }. This is especially useful for converting a series of video lecture episodes into individual M4B chapters.

Technical Notes

M4B is structurally an MPEG-4 Part 14 container (identical to MP4) with an audio-only constraint and the file extension used as a signal to media players to treat it as audiobook content rather than a generic media file. The AAC codec used here is the native and only universally supported codec for M4B — while the format can technically contain MP3 or FLAC streams, Apple devices and most audiobook apps only reliably recognize AAC-encoded M4B files. The -movflags +faststart flag moves the moov atom to the beginning of the file, which enables progressive playback and is recommended for any M4B distributed over the internet or to podcast apps. ID3-style metadata tags (title, author, album, cover art) present in the source MP4 are generally preserved through the conversion since both containers use the same iTunes-compatible metadata atom structure, though cover art handling may vary by source file. Subtitle tracks, which MP4 supports, are silently dropped because M4B has no subtitle track specification. The output file size will be substantially smaller than the input MP4 due to video removal — at 128k AAC, one hour of audio is approximately 56MB.

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