Both play on your iPhone, so the real question is where the file goes next. My short answer: M4A for Apple devices, MP3 when something outside Apple asks for it by name. The reasoning is below, with sources dated.

I build Grabio, a free Shortcut that makes M4A on your phone and sends MP3 jobs to a conversion site. Weigh that, and check the sources yourself.

Short answer

MP3 vs M4A on iPhone: What Each One Is and Which to Pick
  • An iPhone plays both. Apple lists AAC and MP3 on its audio playback line.
  • M4A is a wrapper for MPEG-4 audio, usually AAC. MP3 is a different codec in its own file type.
  • Pick M4A for audio that stays on Apple devices. Pick MP3 when a car stereo, old player or upload form asks for it by name.

M4A and MP3 side by side

At a glance

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M4A and MP3 side by side. Sources: Library of Congress, MPEG-4 File Format, Version 2, Library of Congress, Advanced Audio Coding (MPEG-4), Library of Congress, MP3 (MPEG Layer III) encoding, Library of Congress, MP3 File Format and Apple, iPhone 17 tech specs, all read 4 Nov 2026.
M4A MP3
What the name means A filename extension for an MPEG-4 file that holds audio only A codec (MPEG Layer III) and the file type that wraps it
What is inside Usually AAC. The Library of Congress also notes the extension is used for Apple Lossless. An MP3 bitstream, optionally with ID3 metadata blocks
Lossy or not Depends on what is inside. AAC is lossy compression; Apple Lossless is not. Lossy. It discards or reduces parts of the sound that are less audible.
Quality at the same bitrate The Library of Congress says AAC is significantly better than MP3 at a given data rate, citing commentators, with no test shown The format AAC is compared against
iPhone playback Apple lists AAC and Apple Lossless Apple lists MP3
Reach beyond Apple Not covered by the sources I read Library of Congress: very widely adopted for web distribution and playback on specialized devices
Grabio route Audio to M4A and Extract audio (M4A), both on your iPhone Opens VERT, a separate conversion site, where you choose MP3

What the two names mean

MP3 names two things at once: a codec, MPEG Layer III, and the file type that holds it. The Library of Congress says the codec uses perceptual coding, which discards or reduces parts of the sound that are less audible. It also says an MP3 at 128 kbit/s is about one eleventh the size of an uncompressed CD-quality file.

M4A is different. It is a filename extension for an MPEG-4 file that holds audio only. The Library of Congress page, paraphrasing a former site for the format, says Apple began using M4A to mark files without copy protection.

Inside, you usually find AAC, and the Library notes the extension is also used for Apple Lossless. So “M4A” tells you the wrapper, not whether the audio is lossy.

What is inside an M4A file and an MP3 fileAn M4A file is an MPEG-4 audio container that usually holds AAC and can also hold Apple Lossless.

An MP3 file holds an MP3 bitstream, which is a lossy codec.M4A fileMPEG-4 audio containerAAC (usually)lossy compressionApple Lossless (sometimes)not lossyMP3 fileMPEG Layer IIIMP3 bitstreamlossy, perceptual codingID3 metadata (optional)metadata blocks

The diagram shows the difference. Two M4A files can sound different because they hold different things, while every MP3 holds the same kind of thing at a different bitrate.

Quality, and what I could not source

The Library of Congress says that at a given data rate AAC is significantly better than MP3, and it attributes that to commentators. I could not find a primary test with files and numbers, so treat it as a direction rather than a measurement.

File size is about bitrate, not the wrapper

A song’s size comes from its bitrate and its length. The table below is plain arithmetic, so you can work out your own files. I ignore container overhead and have not measured it.

File size from arithmetic: bitrate times seconds, divided by 8, in megabytes of 1,000,000 bytes. A calculation for a constant bitrate with container overhead ignored, not a measurement of any file.
Bitrate 1 minute 4 minute song 1 hour
128 kbps 0.96 MB 3.84 MB 57.60 MB
192 kbps 1.44 MB 5.76 MB 86.40 MB
256 kbps 1.92 MB 7.68 MB 115.20 MB
320 kbps 2.40 MB 9.60 MB 144.00 MB

If a file is too big to email or message, the bitrate is the lever, and compressing an audio file on iPhone walks through it.

Which one to pick

Which format fits which situation.
If this is you Pick Why, and from where
Audio stays on your iPhone, Mac or iCloud M4A or MP3, either Apple lists AAC and MP3 for playback, so neither needs converting
A car stereo, old player or website asks for MP3 MP3 It is the name those places ask for, and the Library of Congress notes wide adoption. I cannot test your device.
You extract audio from a video on your iPhone M4A Grabio’s Extract audio gives M4A, and it needs no second step
You want the best quality for a given bitrate M4A (AAC) The Library of Congress says AAC beats MP3 at the same data rate. It gives no size figure, so I quote none.
You want lossless M4A with Apple Lossless inside MP3 discards audio by design. Check what your M4A actually holds, since the extension alone does not say.

How to get each on your iPhone

Menu names below come from the current build of the Shortcut.

  1. Add the Shortcut once from the install page.
  2. Share an audio file, or a video, to Grabio from the share sheet.
  3. For an audio file, tap Audio → M4A. For a video, tap Extract audio (M4A).
  4. Grabio converts it on your iPhone and asks where to save it in Files, then shows a Done alert.

For MP3 the route has one more step. Choose Audio → MP3 (open VERT). Grabio opens VERT, a separate conversion site, where you tap Upload, pick the file, choose MP3 and convert. The step-by-step version is in converting audio to MP3 on iPhone and converting M4A to MP3.

Get Grabio, free

A Shortcut for your iPhone share sheet. Share a link to save it, or share a file to convert it on your device. No account.

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More on this silo: converting audio to M4A, extracting audio from a video, M4A to WAV, MP4 to MP3, saving a voice message and recording audio. For how the Shortcut handles files, read how Grabio works.

MP3 vs M4A: FAQ

Does an iPhone play M4A and MP3?

Apple’s iPhone 17 specs list AAC, MP3 and Apple Lossless on the audio playback line. The page names codecs, not file extensions, so it does not spell out the letters M4A.

Is M4A better than MP3?

The Library of Congress says AAC, the usual content of an M4A, is significantly better than MP3 at a given data rate, citing commentators. It shows no test and gives no size figure.

Is M4A lossless?

Not by default. The extension is a wrapper. It usually holds AAC, which is lossy, but the Library of Congress notes it can hold Apple Lossless, which is not.

Is MP3 still worth using?

Yes, when something asks for it. The Library of Congress describes it as very widely adopted for web distribution and for playback on specialized devices. Its patents expired in April 2017, per the Fraunhofer IIS notice the Library cites.

How big is a 4 minute song?

At a constant 128 kbps it works out to about 3.84 MB, and at 320 kbps about 9.60 MB. That is arithmetic, not a measured file. The size depends on the bitrate, not on whether the wrapper is M4A or MP3.

How do I get MP3 from an M4A with Grabio?

Save the M4A to Files, share it to Grabio and choose Audio to MP3 (open VERT). That opens VERT, a separate conversion site, where you pick the file, choose MP3 and convert. It is a second step on a website, not an on-device one.

Aditya R Sharma

Written by

Aditya R Sharma

I build Grabio, a free iOS Shortcut that downloads and converts media entirely on your iPhone. No App Store install, no login, nothing uploaded to a server. I am a GTM builder who writes the code myself, and I write these guides from actually shipping and supporting the thing.

Email meX / TwitterLinkedInadityaarsharma.com

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