Convert M4B to WMA — Free Online Tool
Convert M4B audiobook files to WMA format using FFmpeg in your browser — no upload required. This tool re-encodes the AAC audio track from your M4B into Windows Media Audio (wmav2), making your audiobooks and podcasts compatible with WMA-dependent media players and legacy Windows software.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your M4B file here
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Free — no uploads, no signups. Your files never leave your browser.
Settings
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
M4B files store audio in AAC encoding inside an MPEG-4 container, along with chapter markers and bookmarking metadata. WMA uses Microsoft's proprietary wmav2 codec inside an ASF container, and does not support chapters or audiobook-style bookmarking. During this conversion, FFmpeg decodes the AAC audio stream and re-encodes it as wmav2 at 128k bitrate by default. Because this is a transcode between two lossy codecs — not a remux — there is a small but measurable quality loss as the audio goes through a second lossy compression stage. Chapter markers and bookmarking data from the M4B are discarded, as the WMA format has no equivalent structures for those features. Basic metadata tags such as title and artist are carried over where the ASF container supports them.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg program. In the browser-based version of this tool, this runs via FFmpeg.wasm (WebAssembly) entirely on your local machine — your M4B file is never sent to a server. |
-i input.m4b
|
Specifies the input file — your M4B audiobook. FFmpeg reads the MPEG-4 container and identifies the AAC audio stream and any chapter/metadata structures inside it. |
-c:a wmav2
|
Sets the audio encoder to wmav2, Microsoft's second-generation Windows Media Audio codec. This re-encodes the AAC audio from the M4B into the proprietary WMA format required for .wma output files. |
-b:a 128k
|
Sets the audio bitrate to 128 kilobits per second for the wmav2 output. This is the default balance point between file size and audio quality; for long audiobooks with speech-only content 96k is often sufficient, while music-heavy content benefits from 192k or higher. |
output.wma
|
Defines the output filename and extension. The .wma extension tells FFmpeg to wrap the wmav2 audio stream in an ASF (Advanced Systems Format) container, which is the standard container for WMA files used by Windows Media Player and compatible devices. |
Common Use Cases
- Playing an M4B audiobook on a legacy Windows Media Player setup or a portable device that only supports WMA playback
- Distributing a purchased or self-produced audiobook to a listener who uses a WMA-only in-car entertainment system
- Converting podcast episodes stored as M4B files into WMA for compatibility with older Microsoft Zune devices or Windows Phone players
- Preparing audiobook content for a digital distribution platform or kiosk system that ingests only WMA-formatted audio
- Stripping the chapter/MPEG-4 wrapper from an M4B and producing a flat, single-stream WMA file for simpler linear playback on Windows-centric software
Frequently Asked Questions
No. Chapter markers and audiobook bookmarking are features of the MPEG-4 container that M4B uses, and the WMA/ASF container has no equivalent mechanism to store them. After conversion, you will have a single continuous WMA audio stream with no chapter navigation points. If chapter structure is important, you should split the M4B into individual chapter files before converting.
Yes. Both AAC and wmav2 are lossy codecs, so transcoding between them introduces a second generation of lossy compression artifacts. The original M4B was already lossy; re-encoding it to wmav2 at 128k means audio information lost in the initial AAC encode cannot be recovered, and new wmav2 compression artifacts are added on top. For spoken-word audiobook content this degradation is usually subtle, but it is audible on music or high-fidelity material. Using a higher output bitrate like 192k or 256k minimizes but does not eliminate this loss.
Replace the value after -b:a in the command. For example, to encode at 192k bitrate use: ffmpeg -i input.m4b -c:a wmav2 -b:a 192k output.wma. The WMA format supports bitrates from 64k up to 320k. For voice-only audiobook content, 96k is often sufficient; for higher quality music-heavy content, 192k or 256k is recommended.
Yes, by replacing -c:a wmav2 with -c:a wmav1 in the command: ffmpeg -i input.m4b -c:a wmav1 -b:a 128k output.wma. However, wmav2 is strongly preferred — it is a direct improvement over wmav1 with better audio quality at the same bitrate and is supported by virtually every device and application that plays WMA files. wmav1 is only useful for compatibility with extremely old Windows Media Player versions.
Partially. Basic ID3-style tags such as title, artist, and album stored in the M4B will be mapped to equivalent ASF metadata attributes in the WMA output by FFmpeg. However, M4B-specific fields like narrator, chapter titles, or audiobook-specific iTunes tags may not have direct WMA equivalents and will be dropped. You should verify metadata in your WMA player after conversion.
Not with the single command shown, but FFmpeg supports batch processing through shell scripting. On Linux or macOS, run: for f in *.m4b; do ffmpeg -i "$f" -c:a wmav2 -b:a 128k "${f%.m4b}.wma"; done. On Windows Command Prompt use: for %f in (*.m4b) do ffmpeg -i "%f" -c:a wmav2 -b:a 128k "%~nf.wma". The browser-based tool processes one file at a time, so the FFmpeg command is especially useful for bulk conversions of large audiobook libraries.
Technical Notes
M4B is structurally identical to M4A but carries the .m4b extension as a signal to media players that the file should be treated as an audiobook, enabling chapter navigation and resume-from-bookmark functionality. None of these behavioral signals carry over to WMA, which stores audio in the Advanced Systems Format (ASF) container. The wmav2 codec used in this conversion is Microsoft's second-generation Windows Media Audio codec, offering acceptable quality for speech at 128k but not competitive with modern AAC or Opus at equivalent bitrates. One practical concern with long audiobook M4B files is that they can be several hours long — re-encoding a 10-hour file at 128k wmav2 will take meaningful processing time in the browser via FFmpeg.wasm, and files approaching 1GB may be slow to process. For very large audiobook files, copying the displayed FFmpeg command and running it natively on your desktop will be significantly faster. The -movflags +faststart flag present in M4B output settings is irrelevant to this conversion since the output is WMA/ASF, not an MPEG-4 container.