Opus to M4B Converter

How to Convert Opus to M4B Online

1. Upload Your Opus File: Drag and drop or click "Add Files" to select your.opus recording. Batch is supported — queue an entire podcast season or chapter set and process them together.
2. Pick Quality Preset or Bitrate: Default is the Highest preset, which targets 128 kbps AAC. For spoken-word audiobooks, drop to Medium / Low (64 kbps) — the standard for narration. Power users can switch to Custom Bitrate (enter any kbps), Constant Bitrate (32-384 kbps presets), or Variable Bitrate, or target an exact output via Specific file size in MB/KB.
3. Set Audio Channel and Sample Rate (Optional): Audio Channel defaults to Original — switch to Mono to roughly halve file size for single-narrator content, or force Stereo. Audio Sample Rate also defaults to Original; Opus internally targets 48 kHz, so 44100 Hz or 48000 Hz are the sensible AAC targets (8000-48000 Hz available).
4. Trim and Convert: Toggle Trim on to enter a Start Time and Duration (seconds or HH:MM:SS.sss) — useful for cutting intros or splitting a long recording into per-chapter files. Click Convert. Files are uploaded over an encrypted connection, processed on our servers, and deleted automatically after a few hours — no sign-up, no watermark, never shared.

Why Convert Opus to M4B?

Opus is an IETF-standardized, royalty-free audio codec (RFC 6716, published September 2012) used by default in WhatsApp voice notes, Discord, Zoom, and YouTube audio streams. M4B is Apple's audiobook container: structurally identical to M4A (MPEG-4 Part 14 + AAC), but the.m4b extension signals to Apple Books, Plex, Prologue, and Audiobookshelf that the file is a book — unlocking automatic resume, chapter navigation, and Audiobooks-category placement in libraries.

• Save WhatsApp / Telegram voice notes as listenable books — long voice messages downloaded as.opus files become bookmarked audiobooks that resume where you left off, rather than restarting each time you open the app.
• Convert downloaded podcasts for Apple Books — many podcast apps cache episodes as Opus. M4B puts them in the Audiobooks section of the Books app on iOS, iPadOS, and macOS instead of cluttering the Music library.
• Archive lecture and interview recordings — Opus recorders on Android (Google Recorder, OpenScribe) output.opus by default; M4B lets Apple-ecosystem users resume mid-lecture across devices via iCloud sync.
• Prepare DIY audiobook chapters — split a long Opus recording into individual M4B chapter files using the Trim controls, then merge them in a tool like m4b-tool to produce a single chaptered audiobook.
• Listen on Audiobookshelf, Plex, or Prologue — these open-source audiobook servers detect M4B files and surface bookmark/resume controls automatically; raw Opus files are treated as generic audio with no resume.
• Cut size for offline listening — Xiph.org recommends only 24 kbps mono / 32 kbps stereo Opus for spoken-word content, but Opus → 64 kbps AAC (the audiobook standard) is still roughly a fifth of a typical 320 kbps MP3.

Opus vs M4B — Format Comparison

Bitrate Guide for Spoken-Word Audiobooks

Per Xiph.org's recommended settings, Opus at 24 kbps mono sounds comparable to AAC at roughly 64 kbps — the standard audiobook bitrate. Transcoding to 64 kbps AAC preserves perceived quality with headroom for the lossy-to-lossy step.

Frequently Asked Questions

What is the difference between M4B and M4A?

M4B and M4A use the same MPEG-4 Part 14 container and the same AAC codec — the bytes inside the file are essentially identical. The only difference is the extension:.m4b signals to Apple Books, Plex, Audiobookshelf, and Prologue that the file is an audiobook, which unlocks automatic resume, chapter navigation, and placement in the Audiobooks library. Renaming a.m4a to.m4b is enough to enable audiobook behavior in most players, though the file may still lack embedded chapter markers.

Will converting from Opus to M4B improve the audio?

No — this is a lossy-to-lossy transcode. Both Opus and AAC discard data during encoding, so going Opus → AAC re-encodes audio that was already lossy and can introduce minor artifacts, especially at low bitrates. To minimize loss, start with the highest-quality Opus source you have and encode the M4B at 64 kbps or higher. If you only need a different container with audiobook features and you can find the original master, encoding from that beats transcoding the Opus.

What bitrate should I use for an audiobook?

64 kbps mono AAC is the industry baseline for single-narrator spoken word — clear voice at roughly 28 MB per hour. Bump to 96 kbps stereo for dramatized productions with music and ambient effects, or to 128 kbps stereo for full-cast audiobooks with score. Since most Opus voice recordings cap around 24-32 kbps anyway (per Xiph's recommendations), encoding the M4B above 96 kbps mainly wastes space — you can't add back detail the Opus encoder already removed.

Does M4B work on Android?

Android does not treat.m4b specially out of the box — the default music players ignore the audiobook semantics. However, dedicated audiobook apps handle M4B fully: Smart AudioBook Player, Listen Audiobook Player, Voice Audiobook Player, and the open-source Audiobookshelf mobile app all read chapters and remember position. VLC plays the audio but doesn't resume across sessions.

Why is my.opus file from WhatsApp not opening in iTunes or Apple Books?

Apple's audio stack never adopted Opus playback in iTunes / Apple Books — even though Safari 17+ can decode Opus inside WebM, Apple's media apps don't surface it. That's the most common reason people convert WhatsApp.opus voice notes to M4B: Apple Books accepts the converted file and treats it like any other audiobook, complete with cross-device iCloud resume.

Can I split a long Opus recording into separate chapter files?

Yes. Toggle Trim on, enter a Start Time and Duration for the first chapter, convert, then repeat with the next chapter's start/duration. For a fully chaptered single M4B (where one file contains multiple chapter markers internally), use the per-chapter M4B outputs as inputs to a tool like m4b-tool or AudioBookConverter, which can merge them and embed chapter markers. xconvert's Audio Trimmer is a faster path if you only need to cut, not transcode.

How does this compare to converting Opus to MP3 or M4A?

If your target is a music player, Opus to MP3 is the most compatible choice — every device on the planet plays MP3. For modern Apple devices and lossless-aware players, Opus to M4A gives the same AAC encoding as M4B but without audiobook flags. Pick M4B specifically when you want resume-on-reopen and chapter navigation in Books, Plex, or Audiobookshelf.

Can I convert in the reverse direction?

Yes — M4B to Opus is available for shrinking long audiobooks to roughly half their size for offline listening on Android (Opus is very efficient for speech). You can also convert directly between Opus and Apple's bare AAC stream via Opus to AAC, or compose an audiobook from existing tracks via MP3 to M4B.

Is there a file-size limit and is the conversion private?

Free users can upload audio files up to 1 GB each in a session, and batches process sequentially. files are processed on our servers and deleted automatically after a few hours and aren't shared — you can close the tab to end the session. There's no watermark, no sign-up requirement, and no quality cap on the free tier for this conversion.