Most comparisons of these two start with which one sounds better. That is the wrong first question, because M4A is not a format in the way MP3 is. MP3 names both a container and the codec inside it. M4A names only the container — and the audio inside can be either of two very different things.
The two files that are both called M4A
An M4A holds either AAC, which is lossy like MP3, or ALAC, Apple Lossless, which discards nothing at all. Same extension, same icon, and no way to tell them apart by looking.
You can tell them apart by size. For the same three-minute track:
- AAC at 256 kbps — around 5 MB
- ALAC — around 15 MB, roughly half of what the WAV would be
An iTunes purchase or an iPhone voice memo is almost always AAC. A CD someone ripped in Apple Music with the lossless setting on is ALAC. That difference decides whether converting costs you anything.
Which sounds better
At the same bitrate, AAC generally holds up better than MP3, and the gap widens the lower you go. It is the newer design and it was meant to be MP3's successor. Below 128 kbps the difference is real; at 256 both are past the point where most people hear anything on ordinary equipment.
But this is rarely why anyone converts. Nobody is switching to MP3 for the sound.
Which one actually plays
This is the real difference and the real reason the conversion exists. MP3 plays on essentially everything ever built. M4A does not: older car stereos, cheap MP3 players, some DJ software, some upload forms, and a long tail of software written around MP3 that never learned anything else.
So the honest summary is: AAC is the better codec, MP3 is the one that works. If the thing you are trying to use accepts M4A, keep the M4A — converting it to MP3 costs a little quality and gains you nothing.
When converting is worth it
Convert the files something is actually refusing, and leave the rest alone. Each conversion from AAC to MP3 is a second lossy pass over audio that has already been through one, so at 192 kbps or above the loss stays inaudible, and converting a whole library that already works just makes every file slightly worse.
Two cases where it is clearly worth doing:
- A voice memo you need to send. Speech does not need a high bitrate; 128 kbps is generous. M4A to MP3 handles it, and since voice memos are often personal, it matters that the file never leaves your device.
- An editor that will not read M4A. Then you probably do not want MP3 either — you want uncompressed. M4A to WAV is the one to use, and if the M4A held ALAC, the WAV that comes out is bit-for-bit the original.
One thing no converter can do
Tracks downloaded from Apple Music are protected, and the protection has to be lifted by the account that holds the licence before any tool can read the audio. That is not a limitation of one converter or another; it applies to all of them. Purchased and ripped files are not affected.
Where your M4A probably came from
The origin usually tells you what is inside without opening anything:
- Bought from iTunes — AAC, typically 256 kbps.
- An iPhone voice memo — AAC, at a low bitrate, because it is speech.
- A CD you imported yourself — AAC by default, but ALAC if you turned the lossless setting on. This is the case where converting actually costs you something.
- Exported from an Apple app — usually AAC, and usually generous with the bitrate.
If you are not sure, the file size settles it: an ALAC track is several times larger than an AAC one of the same length.
The rest of the family: .m4b and .m4r
Two more extensions use the same container and confuse people for the same reason. .m4b is an audiobook — identical to an M4A except that players treat it as bookmarkable, so it remembers where you stopped. .m4r is an iPhone ringtone, which is an M4A under 40 seconds with a different extension so that iOS files it in the right place. None of these is a different codec; they are the same box with a different label on the outside.
What happens to the tags
Title, artist and album usually survive the conversion, because MP3 has a matching field in its ID3 tags for each of them. Embedded artwork sometimes does and sometimes does not. Anything Apple-specific — grouping, the parts of an audiobook, play counts, ratings — has nowhere to go and will not come across. If those matter, keep the M4A alongside the MP3 rather than replacing it.
If you do convert, which bitrate to choose is worth two minutes of reading first — it is the only setting that matters, and the default is not always the right one.