Convert WMA to M4B — Free Online Tool
Convert WMA (Windows Media Audio) files to M4B, the MPEG-4 audiobook format, re-encoding the audio from WMA's wmav2 codec to AAC — the native codec for Apple Books, Audible-compatible players, and podcast apps that support chapter markers and bookmarking.
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FFmpeg Command
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DownloadHow It Works
WMA files store audio using Microsoft's proprietary wmav2 (or wmav1) codec inside an Advanced Systems Format (ASF) container. M4B uses an MPEG-4 container with AAC audio — an entirely different codec — so this conversion cannot be a simple remux. FFmpeg must fully decode the WMA audio stream and re-encode it as AAC at the specified bitrate (default 128k). The output is wrapped in an MPEG-4 container with the -movflags +faststart flag applied, which relocates the container's metadata atom to the beginning of the file. This is essential for M4B files used in audiobook and podcast contexts, enabling progressive playback and proper chapter navigation before the entire file has loaded. Because WMA is a lossy format and AAC is also lossy, this is a lossy-to-lossy transcode, meaning some audio generation loss is unavoidable.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg binary — the open-source multimedia processing engine that handles decoding the WMA input and re-encoding it as AAC inside an M4B container. |
-i input.wma
|
Specifies the input WMA file. FFmpeg reads the ASF container and identifies the audio stream encoded with wmav2 (or wmav1), which it will decode to raw PCM audio before re-encoding. |
-c:a aac
|
Sets the audio codec for the output to AAC (Advanced Audio Coding), the native codec for M4B files. This triggers a full decode-and-reencode of the WMA audio stream — it cannot be copied directly because WMA and AAC are incompatible codecs. |
-b:a 128k
|
Sets the AAC audio bitrate to 128 kilobits per second. This is a good general-purpose bitrate for audiobooks and spoken-word content; lower values like 64k are viable for voice-only recordings, while 192k or higher better preserves music or high-fidelity source material. |
-movflags +faststart
|
Moves the MPEG-4 moov atom (the container's structural index) to the start of the output M4B file. This is critical for audiobook playback — it allows apps like Apple Books to begin playback, display chapter navigation, and resume bookmarks before the full file has been loaded. |
output.m4b
|
Specifies the output filename with the .m4b extension. The .m4b extension signals to audiobook-aware players like Apple Books and CarPlay that the file supports bookmarking and chapter navigation, distinguishing it from a standard .m4a audio file even though the container format is identical. |
Common Use Cases
- Converting a WMA-format audiobook purchased or ripped from an older Windows Media Player library so it can be imported into Apple Books with chapter and bookmarking support
- Migrating a large archive of WMA lectures or educational recordings into M4B so students can resume playback mid-file on iPhones and iPads without losing their place
- Preparing a podcast series originally recorded or distributed in WMA format for submission to podcast directories that require AAC-based MPEG-4 audio files
- Re-encoding a WMA audio drama or serialized fiction file into M4B to take advantage of M4B's chapter marker support, enabling scene-by-scene navigation
- Converting WMA files recorded on older Windows devices into M4B so they are compatible with CarPlay audiobook players, which do not support the WMA format
- Stripping DRM-free WMA content from a legacy media library and re-packaging it as M4B to consolidate an audiobook collection into a single Apple Books-compatible format
Frequently Asked Questions
Yes, some quality loss is unavoidable because both WMA and AAC are lossy codecs. When FFmpeg decodes the WMA stream and re-encodes it as AAC, audio artifacts from the original WMA encoding are baked into the source, and AAC encoding introduces its own generation loss on top of that. At the default 128k bitrate, the result is generally transparent for spoken-word audiobook content, but if you have a high-bitrate WMA source (192k or 320k), consider raising the output bitrate to 192k or higher to better preserve audio fidelity.
Yes — this is one of the primary reasons to convert WMA to M4B. WMA's ASF container has no native support for chapters or bookmarking, so long-form content like audiobooks and lectures cannot be navigated by chapter in WMA format. M4B's MPEG-4 container fully supports chapter markers and per-file bookmarking, meaning Apple Books and compatible players will remember exactly where you stopped listening. Note that the FFmpeg conversion itself does not automatically generate chapter markers from a WMA source that had none — you would need to add chapter metadata separately.
FFmpeg will attempt to map standard WMA metadata tags (such as title, artist, album, and year) to their MPEG-4/ID3 equivalents in the M4B output. However, WMA stores metadata in ASF-specific attributes which do not always map one-to-one with MPEG-4 tags, so some fields may be dropped or require manual correction after conversion. Embedded album art is often preserved, but you should verify the output file's tags in a tool like MP3Tag or Kid3 after conversion, especially for audiobook-specific fields like narrator or series.
No. WMA files protected with Microsoft's DRM (Digital Rights Management) cannot be decoded or converted by FFmpeg. The tool will return an error because the encrypted audio stream cannot be accessed without a valid license key tied to the original Windows device or account. Only DRM-free WMA files — such as those ripped from CDs using Windows Media Player or purchased from stores that offered DRM-free downloads — can be converted with this tool.
The audio bitrate is controlled by the -b:a flag in the command. To change quality, replace 128k with your desired bitrate — for example, use -b:a 64k for smaller file sizes suitable for spoken-word content, or -b:a 192k for higher fidelity music. For audiobooks, 64k or 96k mono AAC is often sufficient and produces much smaller files. If your source WMA was encoded at a specific bitrate, there is generally no benefit to targeting an output bitrate higher than the source bitrate.
The -movflags +faststart flag instructs FFmpeg to move the MPEG-4 container's moov atom (the index of the file's structure and metadata) to the beginning of the file after encoding is complete. By default, FFmpeg writes the moov atom at the end. For M4B audiobook files, having the moov atom at the front is important because it allows compatible players — including Apple Books and podcast apps — to begin playback and navigate chapters before the entire file has been read, which is especially valuable for large audiobook files streamed over a network.
Technical Notes
The WMA format stores audio using Microsoft's wmav2 codec (or the older wmav1) inside an ASF container, a format deeply integrated with the Windows Media ecosystem but with limited cross-platform support. M4B uses MPEG-4 Part 14 as its container with AAC (Advanced Audio Coding) as its primary codec, the same codec used across Apple's entire ecosystem and widely supported in Android, CarPlay, and podcast infrastructure. Because this is a codec transcode rather than a remux, processing time scales with file duration. The AAC encoder used here is FFmpeg's native aac encoder, which produces standards-compliant AAC-LC output; for critical listening applications, the libfdk_aac encoder (if available in your local FFmpeg build) may yield marginally better quality at equivalent bitrates, though it requires a separately compiled FFmpeg binary. M4B files produced by this conversion will have a .m4b extension and MPEG-4 container structure identical to .m4a files but flagged for audiobook playback behavior. WMA's metadata model (ASF Content Description and Extended Content Description) differs structurally from MPEG-4's iTunes-style atom-based tags, so tag fidelity after conversion should be verified. WMA's streaming features and DRM capabilities are not applicable to the M4B output format.