Convert M4A to MP3 on iPhone
M4A and MP3 are both lossy. Converting between them throws away audio a second time. Do it when something you own only speaks MP3 — not because you think it will sound better.
What an M4A actually is
M4A is not an audio format. It is a container — an MPEG-4 box structure that holds an audio stream and some metadata. What is inside is almost always AAC, the codec Apple uses for iTunes Store purchases, Voice Memos, and the audio track of every video your iPhone shoots.
Occasionally an .m4a holds ALAC (Apple Lossless) instead. Same extension, completely different animal: ALAC is lossless, AAC is not. If your M4A came from Voice Memos, an iTunes purchase, or a video, it is AAC.
The uncomfortable part: this conversion loses quality twice
AAC is lossy. The encoder analysed the original recording, decided which parts of it your ears would not miss, and permanently discarded them. That audio is gone. It is not recoverable, by any app, on any platform, at any price.
MP3 is also lossy, and it makes its own, different set of decisions about what to throw away. So an M4A to MP3 conversion is a lossy-to-lossy transcode: the MP3 encoder is handed audio that has already been damaged, and damages it again. The result is measurably worse than the M4A, and worse than an MP3 encoded from the original recording would have been.
No converter avoids this. It is a property of the formats, not of the software. Anyone telling you otherwise is selling something.
If you still have the original, encode from that. A WAV, AIFF or FLAC master converted straight to MP3 is a first-generation encode and will sound better than going through an M4A. Only transcode the M4A if the M4A is all you have.
When M4A to MP3 is genuinely the right move
Plenty of the time it is. MP3 is thirty years old and effectively universal, which is exactly why people still need it:
- Car stereos. A 2011 head unit with a USB port will read MP3 off a stick and stare blankly at an M4A.
- Gym and fitness equipment, older MP3 players, dashcams, dictaphones, and a long tail of hardware that was built when AAC was still the new thing.
- DJ software and hardware. Some CDJs and older library tools index MP3 and nothing else.
- Windows and third-party tools that technically support M4A but do it badly, or that reject it at upload.
- Anywhere a form says "MP3 only". Transcription services, podcast hosts, university submission portals, court filing systems.
In all of these, a slightly-degraded file that plays beats a pristine file that does not.
Why MP3 is rare on iPhone (and what Convexy does about it)
Apple has never shipped an MP3 encoder. iOS decodes MP3 perfectly well — it has done since the first iPod — but it cannot create one. In Apple's own audio frameworks, kAudioFormatMPEGLayer3 simply does not appear in the list of formats the system will encode to. There is no setting that turns it on, because there is nothing to turn on.
This is not a licensing problem. The last MP3 patents expired in April 2017; an MP3 encoder costs nobody a cent today. Apple just never added one, presumably preferring you use AAC.
The practical consequence for every iPhone converter app is a fork in the road: bring your own encoder, or send the user's file to a server that has one. A great many apps quietly took the second road. Convexy bundles LAME, the standard open-source MP3 encoder, and runs it on your device. That is why this conversion works with the network switched off.
Pick a bitrate deliberately
Because you are already on the second lossy generation, do not economise here. Give the MP3 encoder room to work:
- 320 kbps — use this. On a transcode, the extra bits are cheap insurance against compounding artefacts.
- 256 kbps — Convexy's default, and fine for speech or casual listening.
- 192 kbps — audible on music if you are paying attention.
- 128 kbps — only when file size genuinely rules, e.g. a long spoken-word recording for a form with an upload limit.
Convexy also exposes sample rate (preserve, 44.1 kHz or 48 kHz) and channels (preserve, mono or stereo). Forcing a voice recording to mono roughly halves the file and costs you nothing, because it was never stereo in any meaningful sense.
How to do it
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Bring the M4A into Convexy
Tap Browse files and pick it from Files or iCloud Drive — or share it into Convexy straight from Voice Memos, Mail or any other app.
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Choose MP3
Convexy only offers the formats your file can actually become, so MP3 will be listed alongside WAV, AIFF, CAF and FLAC.
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Set the bitrate
Open the options and choose 320 kbps for music. Set channels to Mono for a voice recording if you want a smaller file.
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Convert
Encoding happens on your iPhone using the bundled LAME encoder. Nothing is uploaded, and it works in Airplane Mode.
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Save or share it
Rename the output if you want, then save it to Files or send it straight on to wherever it needs to go.
Common questions
Does converting M4A to MP3 lose quality?
Yes, and it is unavoidable. Both formats are lossy. The M4A has already had audio permanently removed by the AAC encoder; the MP3 encoder then removes more, by different rules. Encoding at 320 kbps limits the damage but cannot undo the first generation of loss. Any app that claims a lossless M4A to MP3 conversion is either wrong or lying.
Is MP3 better quality than M4A?
No. At the same bitrate, AAC (what is inside an M4A) generally beats MP3, and the gap widens as the bitrate drops. You convert to MP3 for compatibility, never for fidelity. If a device plays both, keep the M4A.
Why do so few iPhone apps convert to MP3?
Because iOS has no MP3 encoder. Apple's frameworks can decode MP3 but cannot produce it — the MP3 encode format is absent from the system's list of available encoders. An app therefore has to ship its own encoder or upload your file to a server that has one. Convexy ships its own (LAME) and encodes locally.
Can I convert a Voice Memo to MP3?
Yes. Voice Memos records to M4A. Share the memo into Convexy, pick MP3, and set channels to Mono — voice recordings are effectively mono anyway, so this halves the file for no audible cost. The recording never leaves your device, which matters rather a lot for a voice memo.
How big will the MP3 be?
MP3 size is set by bitrate and duration, not by the source. At 320 kbps, expect roughly 2.4 MB per minute; at 128 kbps, roughly 1 MB per minute. If the M4A was encoded at a lower bitrate than the MP3 you choose, the MP3 will be larger than the original while sounding slightly worse — that is normal, and a good reminder that bitrate is a budget, not a quality score.
Does Convexy upload my file to convert it?
No. There is no server to upload to. Conversion runs on your iPhone or iPad, which you can verify by turning on Airplane Mode and converting anyway.