MP4 → FLAC
MP4 to FLAC converter
The MP4’s audio is decoded once and stored losslessly as FLAC, so no further quality is lost; the video is discarded. FLAC cannot restore detail an AAC source already discarded.
…or drag files and folders here. Add as many as you like — everything is processed on this device.
.3g2 .3gp .asf .avi .f4v .flv .m4v .mk3d .mka .mkv .mov .mp4 .qt .weba .webm .wmv
In one paragraph
FLAC is the archival choice for audio you care about: every sample of the decoded track is preserved, but the file is typically half the size of a WAV. Pulling FLAC out of video makes sense for concert footage, recorded performances, camera files that captured PCM audio, and any MKV or MP4 rip whose soundtrack you want to keep in a music library without carrying the picture.
Sonic Batch decodes the audio track in your browser with a WebAssembly build of FFmpeg and encodes FLAC at the bit depth and compression level you pick. Compression level changes only how hard the encoder works and how small the file gets; it never changes the sound, because every level decodes to exactly the same samples.
Does this change the audio quality?
The conversion itself adds no loss. If the video’s audio was PCM, FLAC preserves the original recording exactly. If it was AAC or AC-3, FLAC preserves that decoded audio exactly, including what the original encoder already removed.
- An AAC source does not become higher quality as FLAC; it only stops losing more.
- FLAC is written at 16 or 24-bit. A 32-bit float source is stored at the depth you choose.
- Surround tracks keep all channels unless you choose stereo or mono.
A worked example
Archiving the soundtrack of a recorded concert
- Add the concert MP4 or MKV files.
- Choose 24-bit if the camera recorded PCM; 16-bit is enough for AAC sources.
- Leave the compression level at 5, then add the artist and album in the tag editor.
- Convert and move the FLAC files into your music library.
Use FLAC when you will keep or edit the audio, not just listen
FLAC is the right output if the audio is going into an archive, a music library, or an editing project where every later export should start from the cleanest copy you have. For casual listening on a phone, an MP3 or M4A at 192 kbps is a sixth of the size and sounds the same to most people.
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 |
|---|---|---|---|---|---|
| MP4 | The standard video container. Its audio track is almost always AAC. | Native | Partial | Native | Native |
| MOV | QuickTime video from iPhone and DSLR cameras. Audio is normally AAC or PCM. | Partial | Engine | Native | Native |
| MKV | Open video container used for films, TV recordings and OBS captures. Audio is often AAC, AC-3, DTS, Opus or FLAC. | Partial | Partial | Engine | Engine |
| WebM | Open container used by browser recorders and YouTube downloads. Audio is Opus or Vorbis. | Native | Native | Partial | Partial |
| AVI | Legacy Windows video container from camcorders, older cameras and DivX/Xvid files. Audio is usually MP3, PCM or AC-3. | Engine | Engine | Engine | Engine |
| WMV | Windows Media video in an ASF container. Audio is almost always WMA (Windows Media Audio). | Engine | Engine | Engine | Engine |
| FLV | Flash-era web video, still produced by some stream recorders. Audio is AAC or MP3. | Engine | Engine | Engine | Engine |
| 3GP | Legacy phone recording container, normally AMR-NB or low-bitrate AAC audio. | Engine | Engine | Partial | Partial |
| Output | Encoder | Bitrates / depth | Sample rates | Tags |
|---|---|---|---|---|
| FLAC | flac | 16, 24-bit | 8–96 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 FLAC is bigger than the whole MP4.
That happens when the video was small and its audio was heavily compressed. FLAC has to store every decoded sample, which costs far more than AAC; it is still about half the size of WAV.
My player shows no artwork.
Video files rarely carry album art. Add a cover image in the tag editor before converting and it is embedded in the FLAC.
Safari will not play the file in the browser.
Recent Safari plays FLAC, but older versions do not. The file is valid; open it in a music app or VLC.
Frequently asked questions
Is MP4 to FLAC lossless?
The conversion step is lossless. Whether the result matches the original recording depends on the MP4: PCM audio comes through perfectly, while AAC audio comes through as the compressed version it already was.
Why choose FLAC over WAV?
FLAC holds exactly the same samples in roughly half the space and supports tags and cover art. WAV is better only when a tool refuses FLAC.
Are files uploaded?
No. Everything is processed in your browser tab, and the page has no way to send a file anywhere.
Can I batch convert a folder of videos?
Yes. Drop the whole folder in; each file gets its own row, progress and download, plus a ZIP at the end.