Format
M4A, small audio that plays almost everywhere.
An M4A holds sound compressed with AAC, in the same box structure as MP4 video. It is the usual lossy format on Apple devices and plays on nearly everything else.
In a browser the AAC is written by the browser's own encoder, so results differ between Chrome and WebKit.
Facts
The record.
Extension
.m4a. A file of the same kind may also be .mp4 or .m4b for audiobooks.
Compression
AAC, a lossy codec that is the successor to MP3 and covered by patents, which browsers and operating systems license.
Tags
Title, artist and album are kept when Flex Convert writes the file. The cover picture is not carried over.
Who reads it
Apple Music, iPhones, Android, Windows and most players of the last decade. Some old car stereos know only MP3.
Gapless playback
Flex Convert writes no gapless information, so tracks that run into each other can show a tiny gap.
In Flex Convert
Written from FLAC, MP3, WAV and the sound of videos. AAC sound inside a video is copied out unchanged.
Measured examples
What we measured.
M4A files made on this site from generated FLAC and MP3 files, and copied out of a video.
| Test file | Size | Change | Time | Result |
|---|---|---|---|---|
| 20 s clip, 1080pChrome | 30 MB → 327 kB | 99 percent smaller | 190 ms | Sound stream identical · Copied, no quality lost |
| 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 | |
| 90 s musicWebKit | 2.2 MB → 2.2 MB | 1 percent larger | 4.2 s |
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.
At the 192 kb/s setting, a ninety-second FLAC became 1.7 MB in Chrome and 2.2 MB in WebKit, because the two browsers write different rates: 147 and 192 kb/s. An MP3 of 2.2 MB became 2.2 MB in WebKit. AAC sound already inside a video came out as an M4A in 189 milliseconds with no conversion.
Good to know
What M4A is and is not.
A practical format with two limits worth knowing.
Why the browsers differ
The converter asks the browser to write AAC and the browser decides how. WebKit wrote the rate we asked for on every file. Chrome wrote less on easy sound, because its encoder spends fewer bits where fewer are needed.
Lossy on lossy
Turning an MP3 into an M4A runs two lossy encoders in a row and cannot restore anything. It is useful when a device wants M4A, and it is not an upgrade.
What it costs
About 1.4 MB a minute at 192 kb/s, or 1 MB at 128 kb/s, which is plenty for speech. A CD-quality FLAC is several times larger.
Choosing
When to pick it, and when not to.
A format is a trade. These are the trades this one makes.
Pick M4A when
the file goes to a phone, a car or an app that wants AAC, and size matters.
Pick FLAC or WAV instead when
you are archiving or editing. Never convert M4A back to restore quality.
Pick Opus instead when
you control the player: it is smaller than AAC at the same quality and plays in current browsers and Android.