FLAC to M4B Converter

How to Convert FLAC to M4B Online

1. Upload Your FLAC File: Drag and drop, or click "Add Files" to select. Batch conversion is supported — load each chapter as a separate FLAC and convert them in one session. Only.flac files are accepted on this page.
2. Pick Quality Preset or Custom Bitrate: Default is the Highest preset (AAC ~256 kbps). Drop to Medium or Low for spoken-word audiobooks (64-96 kbps cuts file size by ~60% with no audible loss on speech). For full control, switch to Custom Bitrate and type a value in kbps, choose a Constant Bitrate from 32-384 kbps, or target a Specific file size in MB.
3. Adjust Audio Channel and Sample Rate (Optional): Audio Channel defaults to Original; force Mono for pure narration to halve the bitrate at the same perceived quality, or Stereo for music-rich productions. Audio Sample Rate ranges 8000 Hz through 48000 Hz — keep 44100 Hz for studio FLAC sources, drop to 22050 Hz only if you need every kilobyte.
4. Trim and Convert: Use the Trim controls to set Start Time and Duration in seconds or HH:MM:SS.sss format if you need to clip out silence at the head or tail. Click Convert. Audio processes on our servers — no sign-up, no watermark, no metadata kept on our servers.

Why Convert FLAC to M4B?

FLAC is a lossless codec from Xiph.Org — it preserves every bit of the original PCM down to 4-32 bit depth and sample rates up to 655,350 Hz, which is exactly what you want for archival masters but overkill for listening. M4B is Apple's audiobook container: the same AAC audio you get inside M4A, but with a .m4b extension that tells Apple Books, iTunes, Plex, Prologue, and Audiobookshelf to file it as an audiobook, expose embedded chapter markers, and remember your position when you stop. Converting lossless FLAC to M4B turns archival-quality source material into a portable, bookmarkable file at roughly one-tenth the size.

• Apple Books and iTunes audiobook library — Drop the M4B into iTunes (or Music.app on macOS Catalina+) and it lands under Audiobooks, not Music. The shelf view, sleep timer, and skip-15s controls only light up for .m4b, not .m4a or .flac.
• Lossless concert and lecture recordings — Producers often capture lectures, sermons, and live readings to FLAC for redundancy. Re-encoding to AAC at 64-96 kbps turns a 1.5 GB hour-long FLAC into roughly 28-42 MB without removing anything a listener will hear.
• Multi-hour spoken-word archives — Audiobook narrators and podcast hosts who archive raw takes in FLAC convert finished masters to M4B so the binder client they ship to retailers sees a single chapterable file instead of dozens of FLAC parts.
• Resume playback across devices — M4B's position bookmarking is what makes a 14-hour audiobook listenable. FLAC has no such feature: every player restarts from zero or treats the file as a music track.
• Smaller transfers to phones and watches — A 6-hour audiobook at 64 kbps AAC mono is around 170 MB; the same content in 24-bit/48 kHz FLAC stereo can exceed 4 GB. The M4B fits on an Apple Watch with room to spare.
• Sharing with non-audiophile listeners — Family members rarely have FLAC players installed. Every iPhone, iPad, Mac, and most Android audiobook apps (Smart AudioBook Player, Voice, BookFusion) play M4B natively.

Because FLAC is lossless, the AAC encoder has the full source to work with — every M4B you create from FLAC sounds as good as that bitrate possibly can. Going FLAC→M4B beats MP3→M4B, where you're transcoding a lossy file into another lossy file and stacking artifacts.

FLAC vs M4B — Format Comparison

M4B Bitrate Guide for FLAC Sources

Sizes assume Constant Bitrate; Variable Bitrate output is typically 10-15% smaller for the same perceived quality.

Frequently Asked Questions

Will I lose quality going from lossless FLAC to AAC inside M4B?

Yes, AAC is lossy — but for spoken-word content the loss is essentially inaudible above 64 kbps. AAC-LC at 64 kbps mono is widely considered the sweet spot for audiobooks; commercial Audible downloads ship at 64 kbps mono or 32 kbps depending on the tier. Because your source is lossless FLAC, the AAC encoder starts from a clean signal with no prior compression artifacts, so the M4B sounds as good as AAC at that bitrate can. If the original FLAC contains music or sound effects, jump to 96-128 kbps stereo.

Why M4B instead of M4A — aren't they the same file?

Technically the audio payload is identical: AAC-LC in an MPEG-4 container. The .m4b extension is a hint to the operating system and player. iTunes, Apple Books, and the macOS Music app put .m4a into your music library and .m4b into Audiobooks — separate shelves, separate sleep-timer behaviour, separate position bookmarking. Renaming an .m4a to .m4b works on most players, but if your file already has chapter atoms inside, choosing M4B at conversion time saves you the rename step.

Does M4B carry chapter markers automatically?

The M4B container supports MPEG-4 chapter atoms (the same chpl/text track Apple Books reads), but this converter outputs a single continuous track per input file — it does not auto-detect chapter boundaries from a single FLAC. If you need true chapter navigation, upload each chapter as a separate FLAC and run them in one batch, then combine with a dedicated tool such as m4b-tool or AudioBookConverter that writes a chapter list. The output here will still play in Apple Books, just without internal chapter jumps.

What bitrate should I pick for a 12-hour novel narration?

64 kbps AAC mono is the standard. That's roughly 28 MB per hour, so a 12-hour novel is around 340 MB — comfortable for an iPhone, watch, or Android phone. If you can hear noise or breathiness at 64 kbps (some voices are harder to encode), bump to 96 kbps mono. Reserve 128 kbps stereo for full-cast audio drama or content with significant background music.

Can I convert multiple FLAC chapters into one M4B?

This page converts one input to one output per job, but batch upload is supported — load all chapters and you'll get one M4B per FLAC. To merge them into a single M4B with embedded chapter markers, use a desktop tool that understands chapter generation natively, such as m4b-tool or AudioBookConverter. If you only need a single combined audio file without chapter atoms, concatenate the FLACs in an audio editor first, then convert here. For a smaller lossy intermediate that other audiobook tools accept, see FLAC to MP3.

Will the M4B play on Android?

Yes, but Android does not natively show audiobook-specific behaviour the way iOS does. Apps that understand M4B and its chapter/bookmark atoms include Smart AudioBook Player, Voice (open source), BookFusion, and Plex/Audiobookshelf clients. VLC for Android plays M4B as a regular audio file and remembers position per track, but won't expose the chapter list.

How big will my converted file be compared to the FLAC?

Expect roughly a 10x reduction at 64 kbps mono and 5x at 128 kbps stereo, relative to a typical 16-bit/44.1 kHz stereo FLAC. A one-hour FLAC weighing 300 MB usually becomes about 28 MB at 64 kbps or 56 MB at 128 kbps. The exact ratio depends on the original sample rate and bit depth — a 24-bit/96 kHz studio FLAC compresses more dramatically than a 16-bit/44.1 kHz consumer file.

Can I trim silence from the start or end during conversion?

Yes. Open the Trim panel, set Start Time (where the output begins) and Duration (how long the output runs), in seconds or HH:MM:SS.sss. This is useful for removing the studio count-in, mic-test, or post-roll silence that often sits in raw narrator FLAC takes before mastering. For multi-cut edits on a single file, use Audio Cutter first.

What if I want to keep lossless quality — should I use M4A with ALAC instead?

If lossless audiobook quality matters (rare, but valid for music-heavy productions), use M4A with ALAC (Apple Lossless), not M4B with AAC. The M4B audiobook flags work on both AAC and ALAC payloads in principle, but most converters and players assume AAC inside .m4b. A practical workflow: master to ALAC in M4A for archival, then create a 64-96 kbps AAC M4B for listening — see FLAC to M4A for the lossless path, or MP3 to M4B if your source is already lossy.