AAC to M4B Converter

How to Convert AAC to M4B Online

1. Upload Your AAC File: Drag and drop or click "Add Files" to select your .aac recording. Batch uploads are supported — each file is converted independently into its own .m4b.
2. Pick a Quality Preset or Bitrate: Default is Highest. For spoken-word audiobooks, switch to a Constant Bitrate of 64 kbps (the audiobook industry baseline) or use Custom Bitrate to enter any kbps value. Specific file size lets you target an exact MB output for podcast-host upload caps.
3. Set Audio Channel, Sample Rate, and Trim (Optional): Audio Channel defaults to Original — choose Mono to halve the file size for single-narrator content, or Stereo for productions with music. Audio Sample Rate ranges from 8000 Hz (voice-only) to 48000 Hz. Enable Trim to crop a Start Time and Duration in seconds or HH:MM:SS.sss.
4. Convert and Download: Click Convert. Files process in your browser session — no sign-up, no watermark, no DRM is added.

Why Convert AAC to M4B?

M4B is Apple's audiobook extension for the MPEG-4 container. Technically, an M4B and an M4A file are nearly identical — same AAC codec, same MP4 box structure — but the .m4b extension and ftyp brand signal to media players that this is long-form spoken audio. iTunes, Apple Books, and most third-party audiobook apps respond by enabling features that don't fire on a regular .m4a: bookmarked resume playback, chapter list navigation, variable playback speed presets, and sleep-timer awareness. Re-wrapping a raw AAC stream as M4B is the lightweight step that unlocks all of that.

• Apple Books / iTunes audiobook shelf — Files imported with the .m4b extension are filed under Audiobooks in the Books app instead of Music, so they don't shuffle into your playlists and they remember the last listened-to position across devices via iCloud.
• Long-form recording playback — Lectures, sermons, conference talks, and language-learning material benefit from resume-on-reopen. A 6-hour .aac restarts from the beginning every time you reopen it; the same audio as .m4b picks up where you left off.
• Self-published audiobook delivery — Authors using ACX, Findaway Voices, or direct sales need a chaptered audiobook container. M4B is the most compatible single-file format outside of MP3 splits.
• Podcast archives — Re-encoding a back-catalog of AAC episodes into M4B gives listeners chapter navigation between segments (intro, interview, ads, outro) and a resume-bookmark for long form-shows over an hour.
• Audio diaries and journaling apps — Many iOS journaling and field-recorder apps export .aac. Converting to .m4b makes the recording behave like an audiobook in the Books app — searchable, persistent, and synced across iPhone, iPad, and Mac.
• Smaller file size than MP3 — At the same perceived voice quality, AAC inside an M4B is typically 20-30% smaller than an MP3 of equivalent kbps, which matters when packing dozens of hours onto an older iPhone.

AAC vs M4B Format Comparison

Recommended Bitrate by Content Type

To shrink an already-converted M4B further, run it through Compress M4B. For the reverse direction — stripping the audiobook flag back to a generic music file — use AAC to M4A.

Frequently Asked Questions

What is the actual difference between an M4B and an M4A file?

The codec, container, and bytes inside are nearly the same — both are AAC inside an MP4 wrapper. The difference is the extension and a brand identifier (ftyp) that signals "this is an audiobook." Apple Books, iTunes, and audiobook player apps key off that to enable bookmarking, chapter navigation, and audiobook-shelf placement. A renamed .m4a to .m4b will often work in those apps for the same reason — the audiobook features are unlocked by the extension, not by re-encoding.

Will my chapter markers be preserved if my AAC already has them?

Raw AAC (ADTS) streams generally don't carry chapter markers — the format doesn't have a slot for them. So if your source is a plain .aac file, there are no chapters to preserve. The conversion produces a valid M4B container, but you'll need to add chapter markers afterward with a tool like m4b-tool, ChapterTool, or AudioBookBinder. If your source is actually an MP4-in-AAC-extension file (some exports do this), markers may carry through.

What bitrate should I choose for an audiobook?

64 kbps mono is the standard the ACX (Audible's audiobook program) accepts for narration submission, and it produces about 29 MB per hour. Drop to 32 kbps if you need maximum compression and your content is one narrator with no music. Go to 96-128 kbps stereo if the production includes scored beds, ambience, or multiple voice actors where stereo separation matters.

Can Android phones play M4B files?

Stock Android doesn't treat .m4b as a distinct audiobook format — the file plays as audio in most players, but bookmark-and-resume and chapter navigation depend on the app. Smart AudioBook Player, Voice Audiobook Player, Listen Audiobook Player, and VLC all support M4B with chapters and resume. The plain Google Files audio player and Samsung Music app will play the audio but treat it as a music track.

Does converting from AAC to M4B reduce audio quality?

If you keep the bitrate the same or higher, the conversion is effectively lossless because both formats use the AAC codec — the tool can mux the AAC stream into the MP4 container without re-encoding. If you lower the bitrate (for example, 192 kbps AAC down to 64 kbps M4B), the audio is re-encoded and you'll lose some fidelity. For voice content this is rarely audible; for music it can be.

Can I merge multiple AAC files into one chaptered M4B?

This converter processes each uploaded file individually. To merge a series of chapters into a single M4B with a chapter marker per segment, use a dedicated tool like m4b-tool or AudioBookBinder, which concatenate AAC files and write chapter offsets at each join point. As a workaround here, you can trim sections first and then combine externally.

Is there a maximum length for an M4B file?

The MP4 container itself supports very long files — the practical limit is set by the player. Apple Books reliably handles M4B files well over 24 hours; older iPods had quirks above 13 hours and sometimes required splitting. If you're producing a single-file audiobook over 12 hours, test playback in your target app before distributing.

Will an M4B file work with Audible?

No. Audible distributes its own protected formats (.aax and .aa) with Audible-specific DRM. An M4B file you create cannot be uploaded to a personal Audible library. For self-publishing on Audible, submissions go through ACX as uncompressed WAV or 192 kbps+ MP3 per chapter, not as a packaged M4B.

Does this tool add DRM or watermarks?

No. The output M4B has no FairPlay DRM, no audio watermark, and no metadata pointing back to xconvert. The file you download is identical in playability to one produced by ffmpeg or AudioBookBinder. (FairPlay DRM is only added by Apple at the iTunes Store / Apple Books storefront — third-party tools cannot apply it.)