Upload a AAC file and get a M4A back. No signup, no watermark, no limit on how many.
AAC is MP3's designated successor and wins at low bitrates by a clear margin. A bare .aac file is the raw ADTS stream rather than the .m4a container most software expects.
Where you run into it: YouTube and streaming audio tracks, broadcast, anything that came out of a device rather than a music shop.
What it costs you: The naked .aac extension confuses a surprising amount of software, which is the usual reason people convert it.
M4A is an MPEG-4 container holding AAC audio — the same codec as a bare .aac file, wrapped so that players, tags and cover art all behave.
Where you run into it: iTunes and Apple Music purchases, voice memos, Zoom and Teams recordings, most phone-recorded audio.
What it costs you: Outside Apple's ecosystem support is good but not universal, and a few older tools still refuse the extension on sight.
These two hold the same thing. Your .aac is a raw stream of AAC frames; an .m4a is an MP4 container wrapped around AAC frames — so in principle this is a packaging change and the audio need not be touched at all. This converter does not work that way: it decodes and re-encodes, which costs you one generation. That is a fair trade when the point is to get a file that behaves, but if you want the container without the loss, a remux does it — ffmpeg -i input.aac -c copy output.m4a copies the frames across untouched.
What you actually gain is everything a stream cannot carry. The .m4a reports its duration, so players show a progress bar instead of a question mark; it seeks instantly instead of hunting; it holds artist, album and cover art; and iTunes, Apple Music and the Files app will import it, which they will not reliably do with a bare .aac. If your .aac came from a radio capture or an HLS rip, this is the step that turns it into something your library will accept.
Yes, entirely, and there is no cap on how many files you convert. No account, no watermark, no trial.
One generation's worth, which on this particular pair is avoidable. Because both sides are AAC, a remux would preserve the audio bit for bit; re-encoding at Best quality gets very close but is not identical. From a broadcast or stream capture the difference is not something you will hear — the reason to care is that it is the second encode of a file that already had one.
Almost identical, plus a small amount for the container: an MP4 wrapper adds an index and space for tags, which is a few kilobytes on a song. If you add artwork afterwards it grows by however large the image is, which is usually more than the container itself.
Yes, and you can do it on this page: the converter writes AAC as well, so upload the M4A and choose AAC as the output. There is no separate page for that direction. Both formats are lossy, so every leg re-encodes: going out and coming back costs two more generations, and the damage accumulates.
The file is uploaded to do the conversion — nothing here runs in your browser — and both your upload and the converted result are deleted the moment the file is sent back. Nothing is stored and nothing is logged beyond the request itself.