M4A to M4B Converter

How to Convert M4A to M4B Online

1. Upload Your M4A File: Drag and drop or click "Add Files" to select your .m4a recording. Batch conversion is supported — useful when each M4A is one chapter of a longer book.
2. Pick Quality Preset or Bitrate: Default is Quality Preset (Highest down to Lowest). For spoken word, switch to Constant Bitrate and pick a 32-64 kbps preset, set Custom Bitrate to any value in kbps, target an exact output via Specific file size (MB/KB), or scale proportionally with File Size Percentage.
3. Adjust Audio Channel and Sample Rate (Optional): Audio Channel defaults to Original; switch to Mono to roughly halve the file size for voice content, or keep Stereo for music-heavy podcasts. Audio Sample Rate offers 8000 Hz through 48000 Hz — 22050 Hz or 44100 Hz is plenty for audiobook narration.
4. Trim and Convert: Under Trim, set Start Time and Duration in seconds or HH:MM:SS.sss to drop intros, ad reads, or silent tails. Click Convert. Files process in your browser session — no sign-up, no watermark. Output keeps the AAC codec inside the MPEG-4 container; the .m4b extension is what flips players into audiobook mode.

Why Convert M4A to M4B?

M4A and M4B are the same MPEG-4 Part 14 container with the same AAC audio codec inside — what changes is the extension and how players treat the file. Rename or re-encode an M4A as M4B and Apple Books, the macOS Music app, and dedicated audiobook apps stop filing it under music and start filing it as an audiobook: chapter navigation appears, cover art shows up in the audiobook tile grid, and — most importantly — playback position is remembered between sessions. Open a 12-hour M4B, listen for an hour, close the app, return three days later, and it resumes at the exact second you stopped.

• Resume long lectures and courses — Apple Books and the macOS Music app save the last playback position for .m4b files but not for .m4a files. A 90-minute lecture you have to restart from the beginning every time becomes a file you can pick up wherever you left off.
• Sideload your own audiobooks to iPhone or iPad — Apple sells audiobooks through Apple Books, but it also accepts sideloaded M4Bs synced via Finder (macOS 10.15+) or the Apple Devices app. M4A files sync as music; M4B files land in the Audiobooks library where the playback-speed and sleep-timer controls live.
• Self-published podcasts as binge-listen episodes — Bundling a podcast season into a single M4B with chapter markers (one chapter per episode) is a common request from listeners who want to listen offline without a podcast app eating battery in the background.
• Voice memos turned into journals — iPhone Voice Memos exports .m4a. Converting a series of dated memos to a single M4B with chapter markers turns scattered recordings into a navigable audio diary that Apple Books treats as a book.
• Smaller files for voice content — A 10-hour M4A ripped at 128 kbps stereo is roughly 576 MB. The same content re-encoded as 64 kbps mono M4B is roughly 288 MB with no audible loss for clear speech, freeing iPhone storage for actual music.
• Chapter-aware playback in third-party apps — Smart Audiobook Player, BookPlayer, and Plexamp recognise .m4b and surface chapter lists in their UI. They will play .m4a, but treat it as a single opaque track with no chapter index.

M4A vs M4B — Format Comparison

Audiobook Bitrate Guide (AAC, M4B)

Sizes are AAC at the listed bitrate × 3600 s ÷ 8 bits/byte; round to the nearest MB.

Frequently Asked Questions

If M4A and M4B contain the same AAC audio, why bother converting?

The audio data is identical at the same bitrate, but the .m4b extension is what tells iOS, macOS, and audiobook apps to enable audiobook behaviour — playback-position memory, chapter navigation in the dedicated UI, and filing under Audiobooks instead of Music. Without that extension, a 10-hour file behaves like a 10-hour song: no resume, no chapter list, no audiobook speed controls.

Can I just rename .m4a to .m4b instead of converting?

For a single file with no chapter markers, renaming works on macOS and iOS — both rely on the extension to decide whether the file is a song or an audiobook. Re-encoding through this converter is worth doing when you also want to change bitrate (e.g., shrink a 256 kbps music-grade M4A to 64 kbps mono for narration), switch from stereo to mono, trim out silence or ad reads, or hit a specific target file size. Renaming alone does none of that.

Does this converter add chapter markers automatically?

No — chapter markers require timestamped boundaries that only you know (each chapter's start time, optional title, optional cover image). This tool re-encodes a single M4A into a single-chapter M4B; combining multiple recordings into a multi-chapter book is a separate workflow. If chapters matter, name your files in playback order and convert them individually, then assemble with a chapter-aware tool such as the open-source AudioBookConverter or Audiobook Binder on macOS.

What bitrate should I pick for a spoken-word audiobook?

64 kbps AAC mono is the practical sweet spot for narration — about 29 MB per hour, indistinguishable from higher bitrates on phone speakers or earbuds for clear speech. Drop to 32-48 kbps mono only if storage is tight or the source is already low quality. Go to 96-128 kbps stereo only when there is genuine stereo content (sound design, music beds, dramatised audio). Higher than 128 kbps for pure narration is wasted bytes.

Will the converted M4B play on Android?

Android does not treat .m4b as a special format the way iOS does, but the audio inside is standard AAC in MP4, which Android plays natively. Generic music players will play it as one long track; for audiobook behaviour (chapter list, resume position) install a dedicated app such as Smart Audiobook Player, Voice Audiobook Player, or VLC, all of which recognise .m4b. The file plays everywhere; the audiobook UI is what varies.

How do I get the converted M4B into Apple Books on iPhone?

On macOS Catalina or later, drag the .m4b into the macOS Books app, then sync your iPhone or iPad through Finder (or Apple Devices on Windows) with Books sync enabled. Alternatively, AirDrop the file from a Mac and tap "Open in Books" on iOS. The file appears in the Audiobooks tab — not the Reading Now home tab — with playback position synced between devices that share the same Apple ID.

Will DRM-protected Audible or Apple Books files convert through this tool?

No. Audible's .aax/.aaxc files and DRM-protected Apple Books purchases are encrypted and refuse to decode without an authorised account. This converter handles only unprotected M4A — typically Voice Memos exports, podcast downloads, your own recordings, or M4A files you've ripped from non-DRM sources. Trying to convert a protected file will fail at the decode step.

Can I shrink a long audiobook to fit a target storage limit?

Yes. Choose Specific file size and enter the cap in MB (e.g., 250 MB for a 10-hour lecture). The converter picks a bitrate that lands close to that cap. If the result still feels too large, switch Audio Channel to Mono — that alone roughly halves the size for narration. The bitrate-guide table above is the quickest way to estimate before you upload.

Need the reverse or a different starting format?

If you want to go back to a regular music-style track, use M4B to M4A. To build an M4B from an MP3 source instead, use MP3 to M4B. To trim out an intro, ad read, or silent tail without re-encoding the whole file, use Trim M4B on the output.