FLAC to M4A
Turn FLAC into a small M4A.
A lossless FLAC is great at home and heavy on a phone. Make a lossy M4A copy at the quality you choose and keep the FLAC where it is.
Needs a browser that can make M4A sound itself. Tested in Chrome and in WebKit, the engine behind Safari; Firefox is not tested.
Drop a song here.
Drop a song here.Choose a song.
MP3WAVFLACM4ANothing leaves this device.
Several files of one kind convert together.
How it works
Pick the quality, convert, copy to the phone.
1
Drop the FLAC files
One track or a whole album. They run one after another.
2
Pick the bitrate
The default, Plays everywhere, writes 192 kb/s. Small file uses 128, which saves space for speech and podcasts, and Best in Custom uses 320 for critical listening.
3
Save
M4A files with title, artist and album carried over from the FLAC. The cover picture is not carried over.
Measured examples
The browser writes the AAC, and browsers differ.
Three generated FLAC files went through this page at its setting, 192 kb/s, in Chrome and in WebKit.
| Test file | Size | Change | Time | Result |
|---|---|---|---|---|
| 90 s musicChrome | 2.5 MB → 1.7 MB | 34 percent smaller | 770 ms | |
| 90 s musicWebKit | 2.5 MB → 2.2 MB | 14 percent smaller | 2.3 s | |
| 60 s noiseChrome | 6.6 MB → 1.2 MB | 81 percent smaller | 1.2 s | |
| 60 s noiseWebKit | 6.6 MB → 1.5 MB | 78 percent smaller | 3.4 s | |
| 90 s music with tags and coverChrome | 2.6 MB → 1.7 MB | 34 percent smaller | 770 ms | Title, artist and album kept · Cover not kept |
| 90 s music with tags and coverWebKit | 2.6 MB → 2.2 MB | 15 percent smaller | 2.3 s | Title, artist and album kept · Cover not kept |
Measured on November 7, 2026 in Google Chrome 157, WebKit (Playwright build, not Safari) 26, on Apple M1 Pro. Times run from pressing Convert to the result being ready, with the page already loaded. Likeness is PSNR against the original: higher is closer, and above 40 dB the difference is hard to see. WebKit here is Playwright's build, which is not Safari. Every test file was generated for these measurements, none is anyone else's work, and each is described on the About page.
We asked for 192 kb/s. WebKit wrote 192 kb/s on every file. Chrome wrote 147 kb/s on the chord pad and 164 kb/s on the noise, because its encoder spends less on easy sound. The same FLAC therefore came out 1.7 MB in Chrome and 2.2 MB in WebKit. Title, artist and album carried over. The cover picture did not.
Facts
What the M4A keeps.
Tags
Title, artist and album carried over in both browsers.
Cover picture
Not carried over. Our test FLAC had a JPEG cover and the M4A has none: add it again in a tag editor.
Length
About 22 milliseconds longer than the FLAC in both browsers: the encoder's start delay.
Rate and channels
44.1 kHz stereo stayed 44.1 kHz stereo. Higher rates are resampled for AAC.
Good to know
What the lossy copy costs.
AAC is a lossy step. The numbers below let you choose with open eyes.
Size by bitrate
At 192 kb/s a minute of stereo is about 1.4 MB, so a four-minute song is under 6 MB. At 320 kb/s it is 2.4 MB per minute, and at 128 kb/s 1 MB. A CD-quality FLAC is several times larger, typically 5 to 7 MB per minute depending on the music.
The rate you get depends on the browser and the sound, as the table shows. Mono sources are capped at 192 kb/s, since AAC does not need more for one channel.
What changes in the sound
24-bit and 96 kHz files are resampled to 44.1 or 48 kHz for AAC. A 5.1 FLAC is mixed down to stereo, with a limiter so loud passages do not clip. Plain stereo music is the case this page is built for.
Not for gapless albums
The AAC encoder in a browser adds about 44 milliseconds at the start, and no gapless information is written. Continuous albums, live recordings and classical works with tracks that run into each other can show a tiny gap between M4A tracks. Keep those as FLAC, or use a player with gapless support for lossless.
FLAC in Safari
Safari's own decoder failed on FLAC files in our WebKit tests, so Flex Convert decodes FLAC with its own small decoder when the browser's fails. It matched a reference decoder bit for bit across 20 test files. Never convert M4A back to FLAC to restore quality; it only makes the lossy file bigger.
Opus instead of M4A?
Opus is better than AAC at the same bitrate and is available under Custom, as Compact. The catch is playback: it plays in current browsers and on Android, but older phones and some car systems may not. Choose M4A when the file has to work everywhere, Opus when you control the player.
If something goes wrong
When it does not work.
Three things people notice after converting a library.
The cover art is gone
Why: The cover picture is not carried from the FLAC into the M4A.
What to do: Add the cover again with a tag editor, or keep the FLAC for the player that shows it.
A tiny gap between tracks
Why: The browser's AAC encoder adds a start delay and writes no gapless information.
What to do: Keep continuous albums and live recordings as FLAC, or use a player with gapless support for lossless.
The file is smaller than 192 kb/s suggests
Why: Chrome's encoder spends fewer bits on easy sound: 147 kb/s on our chord pad.
What to do: Nothing to fix. If you need the full rate, use WebKit or choose 320 kb/s in Custom.