Convert MKV to M4B — Free Online Tool
Convert MKV files to M4B audiobook format, extracting the AAC audio stream into an MPEG-4 container with chapter markers and bookmarking support. Ideal for turning video lectures, recorded audiobooks, or multi-track MKV files into a format natively recognized by Apple Books, iPhone, and podcast apps.
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FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
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Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
MKV is a flexible container that can hold video, multiple audio tracks, subtitles, and chapters. M4B is an audio-only MPEG-4 container designed specifically for audiobooks and podcasts — it has no video track at all. During this conversion, the video stream is discarded entirely, and the audio is encoded to AAC at 128k bitrate using FFmpeg's native AAC encoder. If your MKV already contains an AAC audio track, the quality loss is minimal but a re-encode still occurs to ensure M4B container compatibility. Chapter metadata from the MKV is preserved in the output M4B, which means apps like Apple Books and Overcast can display chapter navigation. The -movflags +faststart flag reorganizes the MP4 atom structure so the file can begin playing before it finishes downloading — important for podcast and streaming use cases.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg binary. When run via this browser tool, it executes inside a WebAssembly sandbox using FFmpeg.wasm — no files leave your device. The command shown is also valid for running locally on any desktop installation of FFmpeg. |
-i input.mkv
|
Specifies the input Matroska file. FFmpeg reads the MKV container and identifies all internal streams — in this case, at minimum a video stream and at least one audio stream, plus any chapters, subtitles, or metadata embedded in the file. |
-c:a aac
|
Encodes the audio stream using FFmpeg's native AAC encoder, which is the required audio codec for M4B audiobook files. AAC provides better quality than MP3 at equivalent bitrates and is natively supported on all Apple devices and most modern media players. |
-b:a 128k
|
Sets the AAC audio bitrate to 128 kilobits per second. This is the default and produces clear, intelligible audio for speech and audiobook content. For music-heavy content or higher fidelity, this can be increased to 192k or 256k directly in the command. |
-movflags +faststart
|
Relocates the MPEG-4 moov atom to the start of the output file, enabling progressive playback and streaming. This is particularly important for M4B files distributed as podcasts or hosted online, as it allows playback to begin before the full file has downloaded. |
output.m4b
|
Defines the output filename with the .m4b extension, which signals to Apple Books, iTunes, Overcast, and compatible podcast apps that this is a bookmarkable audiobook file. The .m4b extension is what enables chapter navigation and playback position memory on supported platforms. |
Common Use Cases
- Convert a recorded video lecture series (saved as MKV) into an M4B audiobook so you can listen on your iPhone with chapter-by-chapter progress bookmarking
- Turn a multi-chapter MKV recording of a DRM-free audiobook into an M4B file that Apple Books recognizes natively with proper chapter navigation
- Strip the audio from an MKV video podcast recording to produce an M4B episode that retains chapter markers for podcast app compatibility
- Convert a foreign-language learning video course stored as MKV into an M4B so you can practice listening during commutes without the video component
- Package a recorded interview or panel discussion from MKV into M4B format for distribution as a bookmarkable podcast episode
- Extract the audio narration track from an MKV documentary or educational video to create a portable M4B file for offline listening on Apple devices
Frequently Asked Questions
Yes — both MKV and M4B support chapter metadata, and FFmpeg maps MKV chapter data into the M4B container's chapter atom structure during conversion. This means apps like Apple Books, Overcast, and Pocket Casts will display chapter titles and allow you to jump between them. However, if your MKV has no embedded chapters, the M4B will also have none — the conversion cannot generate chapter markers that didn't already exist in the source file.
Yes, the audio is always re-encoded in this conversion, even if your MKV already contains an AAC track. This is because the AAC stream must be remuxed into an MPEG-4 container with specific atom layout requirements, and FFmpeg's safest path is a full encode. The default 128k bitrate produces very good quality for speech-based content like audiobooks and lectures. For music or high-fidelity content, consider increasing the bitrate to 192k or 256k in the FFmpeg command by changing -b:a 128k accordingly.
Both are dropped entirely. M4B is an audio-only format and has no mechanism for storing video streams or subtitle tracks. Only the first audio track is encoded into the output — if your MKV contains multiple audio tracks (e.g., director's commentary or a dubbed language), only the default audio stream is used. If you need a specific audio track, you can modify the FFmpeg command to target it using -map 0:a:1 (for the second audio track) before the output filename.
By default, FFmpeg selects the first (or default-flagged) audio track from the MKV. To target a specific audio track, add -map 0:a:1 to the command to select the second audio track, or -map 0:a:2 for the third, and so on. For example: ffmpeg -i input.mkv -map 0:a:1 -c:a aac -b:a 128k -movflags +faststart output.m4b. You can inspect which audio tracks exist in your MKV beforehand using ffprobe or by running ffmpeg -i input.mkv without an output argument.
The +faststart flag moves the MP4 moov atom (the index that describes the file's structure) to the beginning of the file rather than the end. Without it, podcast apps and streaming players must download the entire file before playback can start. For audiobook and podcast use cases — which is exactly what M4B is designed for — this flag is strongly recommended. You can technically remove it if you are only playing the file locally from disk and never streaming it, but there is no practical reason to do so.
To change audio quality, replace 128k in -b:a 128k with your preferred bitrate — 192k or 256k is recommended for music content, while 96k is sufficient for voice-only audiobooks. To batch convert all MKV files in a directory on Linux or macOS, use: for f in *.mkv; do ffmpeg -i "$f" -c:a aac -b:a 128k -movflags +faststart "${f%.mkv}.m4b"; done. On Windows Command Prompt, use: for %f in (*.mkv) do ffmpeg -i "%f" -c:a aac -b:a 128k -movflags +faststart "%~nf.m4b".
Technical Notes
M4B is structurally an MPEG-4 Part 14 container (identical to MP4) with the file extension signaling audiobook intent to compatible apps — Apple Books, iTunes, and most podcast players treat .m4b files as bookmarkable audio, whereas .m4a files with identical content may not receive the same treatment. The only audio codecs valid in this container for broad compatibility are AAC variants; the default AAC encoder used here is FFmpeg's built-in native AAC encoder, which produces good results but is considered slightly inferior to the libfdk_aac encoder at equivalent bitrates (libfdk_aac is not included in most pre-built FFmpeg distributions due to licensing). MKV's subtitle tracks (ASS, SRT, PGS) and video streams are silently discarded since M4B has no provision for them. ID3-style metadata tags such as artist, album, and title can be embedded in M4B and will be preserved if they exist in the MKV's metadata atoms, though MKV uses a different tagging scheme (Matroska tags) that FFmpeg translates on a best-effort basis. Files with variable-bitrate audio or exotic MKV audio codecs like TrueHD or DTS will be fully transcoded to AAC, which may take noticeably longer than converting a standard AAC or MP3 source track.