FLAC → M4A
FLAC to M4A converter
FLAC is decoded to its original samples and encoded once to AAC inside an M4A. Starting from lossless means one lossy generation rather than two, but AAC still discards information.
…or drag files and folders here. Add as many as you like — everything is processed on this device.
.aif .aifc .aiff .flac .wav .wave
In one paragraph
The Music app on a Mac and iTunes on Windows will not import FLAC, and neither will the iPhone’s music library. If your collection is FLAC and you want it on Apple devices, converting a copy to AAC in an M4A is the standard answer — it is the same codec Apple sells music in, at 256 kbps for iTunes Plus.
Because the source is lossless, the AAC encoder works from the original samples rather than another codec’s artefacts, so this is the best-case lossy encode. Sonic Batch decodes the FLAC and encodes AAC-LC with FFmpeg in WebAssembly, maps the Vorbis comments to iTunes tags and carries the cover art into the M4A. Keep the FLACs as your masters.
Does this change the audio quality?
AAC is lossy, so some information is discarded. From a FLAC source it is a single, first-generation encode: at 256 kbps the result is very hard to tell from the original on almost any material.
- This page writes AAC, not Apple Lossless (ALAC). If you need lossless playback in Apple Music, AAC is not that.
- The encoder is FFmpeg’s native AAC-LC, not Apple’s own. At 192 kbps and above the difference is not audible; at low bitrates Apple’s encoder is ahead.
- AAC tops out at 48 kHz here. Hi-res 88.2 or 96 kHz FLACs are resampled to 48 kHz, and the per-file notes say so.
A worked example
Putting a FLAC album on an iPhone
- Drop the album’s FLAC files into the queue in track order.
- Leave the bitrate at 256 kbps, the same rate Apple uses for its own downloads.
- Keep cover art on so the album artwork shows in the Music app.
- Convert all, download the ZIP and add the M4A files to your Music library to sync.
256 kbps for music, 128 kbps for spoken word
For music you will listen to closely, 256 kbps AAC is the long-standing choice: it is what Apple distributes and it leaves no practical margin for audible artefacts. 192 kbps is a sensible saving for a phone with limited space. Audiobooks and lectures sound fine at 96–128 kbps, especially with Channels set to Mono. Keep the FLACs, so a different bitrate later is a fresh encode from the master.
Supported formats, limits and browser notes
Sonic Batch decodes with the browser when it can (faster, no engine download) and falls back to the self-hosted WebAssembly engine when it cannot. Every combination below works in every listed browser; the column only tells you which decoder does the work.
| Input | What it is | Chrome | Firefox | Safari | iOS |
|---|---|---|---|---|---|
| FLAC | Lossless compression, typically 50–60% of WAV size, bit-identical when decoded. | Native | Native | Native | Native |
| WAV | Raw PCM samples with a small header. Perfect fidelity, roughly 10 MB per stereo minute. | Native | Native | Native | Native |
| AIFF | Apple’s uncompressed PCM format — the WAV equivalent for Logic and Pro Tools. | Partial | Partial | Native | Native |
| Output | Encoder | Bitrates / depth | Sample rates | Tags |
|---|---|---|---|---|
| M4A | aac | 32–320 kbps | 8–48 kHz | Yes + artwork |
- Max per file
- 400.0 MB
- Max per queue
- 4.00 GB
- Max duration
- 4:00:00
- Max archive
- 1.50 GB
These are enforced ceilings, checked before any decoding starts, not measured maximums — a file over the limit is rejected rather than allowed to exhaust the tab’s memory. Real-world capacity is lower on older phones, which is why the scheduler runs one file at a time on iOS and Safari.
Where your audio goes
Nowhere. Your file is read from disk into this browser tab with the File API, decoded and encoded by code running in that same tab, and handed back to you as a download. It is never transmitted.
That is a structural property of the application, not a policy promise: there are no API routes, no server actions and no third-party processing services in this project, so there is nothing capable of receiving a file. The page’s Content-Security-Policy also restricts connections to this origin, and the automated test suite includes a browser test that fails if any network request during import, preview, conversion or download carries media bytes.
The trade-off, stated plainly: results live in your tab’s memory until you download them. Closing or reloading the page discards them. There is nothing on a server to recover, because nothing was ever put there.
Troubleshooting
The album art did not appear in Apple Music.
Keep cover art must be on, and the FLAC must contain an embedded picture — some rips keep cover.jpg as a separate file. The source panel shows whether artwork was found.
Track numbers or album artist are missing.
FLAC tags are free-form Vorbis comments. Standard names such as TRACKNUMBER and ALBUMARTIST map across; unusual field names do not. Fill gaps in the metadata controls before converting.
A 96 kHz file came out at 48 kHz.
That is expected. The AAC encoder used here supports up to 48 kHz, and 48 kHz already covers the full audible range.
Frequently asked questions
Does FLAC to M4A lose quality?
Yes, a little: AAC is lossy. Encoding once from a lossless source at 256 kbps keeps that loss below what almost anyone can hear.
Why not convert FLAC to MP3 instead?
Both work on Apple devices. AAC sounds better than MP3 at the same bitrate and M4A handles album artist, disc numbers and artwork more cleanly. Use the FLAC to MP3 page for car stereos and older players.
Can I convert my whole FLAC library?
Yes. Add folders, apply one setting to everything, and download as a ZIP or as individual files. Very large libraries are easiest in batches of an album or two.
Are my files uploaded?
No. The FLAC decoder and AAC encoder run as WebAssembly in your browser tab, served from this domain. Nothing is transmitted.