Convert MP3 to M4A on iPhone
MP3 and M4A are both lossy. Converting one to the other re-encodes already-damaged audio, so the result is slightly worse than what you started with. There are still good reasons to do it.
What M4A is
M4A is a container, not a codec — an MPEG-4 box structure holding an audio stream. What Convexy puts inside it is AAC, the codec Apple has standardised on for iTunes purchases, Voice Memos, and the soundtrack of every video an iPhone records.
AAC is a genuinely better codec than MP3. It was designed a decade later, it is more efficient at every bitrate, and the advantage grows as the bitrate falls — a 128 kbps AAC comfortably beats a 128 kbps MP3. That fact is what makes this conversion tempting, and also what makes it misleading.
A better codec cannot fix a worse file. Your MP3 has already had audio permanently removed. Re-encoding it as AAC hands the AAC encoder that damaged audio and asks it to compress the damage efficiently — which it does. The output is not better than the MP3. It is a second lossy generation, and measurably slightly worse.
When it is still the right call
Plenty of the time. The quality difference from one careful transcode at a sensible bitrate is small, and the practical benefits are real:
- The Apple ecosystem. M4A is what iOS, macOS, Apple Music, GarageBand and the share sheet expect. Everything simply works better.
- Ringtones and alerts, which need AAC-family audio, not MP3.
- Chapters, cover art and rich metadata, which the MPEG-4 container handles far more cleanly than MP3's bolted-on ID3 tags.
- A smaller file at similar quality — genuinely true for a first-generation encode, and roughly true even for a transcode if you are not going below the source bitrate.
- A device, app or upload form that wants M4A or AAC and will not take MP3.
The rule that actually matters: go back to the source
If the MP3 was made from a CD, a WAV, an AIFF or a FLAC that you still have, convert from that instead. A first-generation AAC from a lossless master is better than the MP3 and better than any M4A you can make from the MP3 — by a margin that is not subtle.
Transcode the MP3 only when the MP3 is genuinely all you have. That is the whole rule, and it applies to every lossy-to-lossy conversion there is.
How Convexy writes the M4A
Convexy decodes your MP3 and re-encodes it to AAC on the device, using Apple's own AAC exporter at its standard high-quality setting. There is no bitrate dial for M4A output — Apple's exporter does not expose one — so what you get is a good, conservative AAC encode rather than a set of knobs to get wrong.
Everything happens on your iPhone. There is no upload, no account, and no server involved at any point, which you can confirm by doing it in Airplane Mode.
One thing this does not do
It does not make the file lossless. An .m4a can contain ALAC (Apple Lossless) instead of AAC — same extension, different codec — and people occasionally assume that is what they are getting. Convexy writes AAC, and in any case wrapping a decoded MP3 in a lossless codec would only produce a larger file containing exactly the same damaged audio. There is no path from a lossy file back to the original recording. There never has been.
How to do it
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Bring the MP3 into Convexy
Tap Browse files and pick it from Files or iCloud Drive, or share it in from another app.
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Choose M4A
Convexy only offers formats your file can actually become — M4A, WAV, AIFF, CAF and FLAC.
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Convert
The MP3 is decoded and re-encoded to AAC on your device using Apple's audio frameworks. Nothing is uploaded.
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Save or share it
Rename the output if you need to, then save it to Files or send it on to wherever it is going.
Common questions
Is M4A better quality than MP3?
As a format, yes — AAC is more efficient than MP3 at every bitrate, and much more so at low ones. As a conversion, no. Converting an existing MP3 to M4A cannot recover what the MP3 encoder deleted; it just re-encodes the survivors. The result is a second lossy generation and slightly worse than the MP3. Format quality and file quality are not the same thing.
Will the M4A be smaller than the MP3?
Usually somewhat, because AAC packs the same audio into fewer bits. Do not treat the saving as free — it is being paid for with a second round of lossy encoding. If size is the goal and quality matters, go back to the original recording and encode once.
Why won't my MP3 work as an iPhone ringtone?
Because iOS ringtones use the AAC family (an .m4r file, which is an M4A under a different name), not MP3. Converting to M4A is the first step. It is one of the clearest cases where the format, not the fidelity, is the entire point.
Should I convert my whole MP3 library to M4A?
No. Every track would take a second lossy hit for no quality gain and a modest size saving. If your library plays fine, leave it alone. Convert individual files when something specifically needs M4A, and if you ever re-rip from CDs or lossless sources, encode to AAC directly from those.
Does this happen on my device?
Yes. Convexy has no server. The decode and re-encode run on your iPhone or iPad, and the conversion works with the network switched off entirely.