Convert M4A to MP3 on Mac

Measured on a 6,214,956-byte, 234-second M4A carrying six text tags and a 37,970-byte JPEG cover. Machine: M2 Pro, macOS 27.0 (build 26A428), 26 September 2026.

Read that last row carefully. We make Smol, and on the single most-requested feature in this category it loses to a free tool. That is the honest state of it and the rest of this page is written accordingly.

Apple documents the whole thing in the Music User Guide, and the steps have barely changed since iTunes:

1. In Music, choose Music → Settings, click Files, then Import Settings.
2. In the Import Using pop-up, choose the format you want. Here, MP3 Encoder.
3. Select one or more songs, then choose File → Convert → Create MP3 Version.

Apple is explicit about the outcome: “The song in its original format and the newly converted song appear in your library.” Nothing is replaced, so the worst case is two copies and a tidy-up. One documented limit worth knowing: “You can convert iTunes Store purchases only if they’re iTunes Plus songs.”

Because Music writes the MP3 from its own library record, the tags and the artwork you see in Music are what land in the file. That is the real reason this route is the default recommendation: it is not faster or smaller, it is that the library is the source of truth for the metadata.

We have not published measurements for it. Doing so would mean importing test files into a real Music library, and a figure we did not measure cleanly is worth less than saying so.

No. macOS ships an MP3 decoder, not an MP3 encoder, and afconvert is a front end to Core Audio’s codecs. The format appears in afconvert -hf, which is what sends people down this path, and then the conversion fails:

$ afconvert -f MPG3 -d '.mp3' master.m4a out.mp3
Error: ExtAudioFileSetProperty ('cfmt') failed ('fmt?')

No output file is written. This is not a version quirk; it is the state of Core Audio on macOS 27.0.

What afconvert is genuinely good at is everything adjacent. It encoded our test file to AAC, ALAC, FLAC and WAV without complaint, it is installed on every Mac, and it needs no Homebrew. If your actual requirement is “a smaller file that plays everywhere on Apple hardware” rather than literally MP3, this is a one-liner:

afconvert -f m4af -d aac -b 128000 input.m4a output.m4a

With one important caveat that cost us a measurement to find: afconvert writes out no metadata at all. Across fourteen tagged test files, every AAC, ALAC and FLAC file it produced came back with zero tags and zero embedded artwork. Not a stripped cover, not a truncated one. Nothing. If you script a library conversion with afconvert, you will end up with a folder of correctly encoded, completely anonymous audio.

The one everyone copies is ffmpeg -i input.m4a output.mp3, and it is worth understanding what that actually does before you run it over a library. This is the subject of the most-viewed question on the topic: “FFmpeg: Convert m4a to mp3 without significant loss” on Super User, asked 20 January 2014, score 153 with 223,799 views and seven answers as of 26 September 2026.

The default is 128 kbps constant bitrate, measured at exactly 128,000 bps on the output stream. Handed a 212 kbps AAC file, it quietly hands you back something worse, which is the “significant loss” in that title.

And there is the artwork problem:

ffmpeg preserves the cover, and then re-encodes the JPEG as a PNG to store it, which on our fixture turned 37,970 bytes of artwork into 280,962. Add -c:v copy and the original JPEG is carried through byte for byte. Across a fourteen-file batch that single flag was the difference between 10,538,345 bytes of output and 7,136,471.

The command worth keeping, with a sane bitrate and the artwork left alone:

ffmpeg -i input.m4a -map 0 -c:a libmp3lame -q:a 2 \
       -c:v copy -id3v2_version 3 output.mp3

-q:a 2 is LAME’s V2 variable bitrate, which spends bits where the music is and drops through quiet passages. Our alert suite is mostly silence, so it averaged 70,634 bps and produced 2,105,392 bytes; the dense tone bed averaged 105,361. Your own music will land far higher. -id3v2_version 3 is there because a few older players and car stereos ignore ID3v2.4 tags entirely.

Speed across many files, and one fewer thing to remember. Smol converted the same file to MP3 in 1,881 ms, 6,214,956 bytes down to 2,067,398, a 66.7% cut. On the dense fixture it was 5,727,250 to 2,371,750 in 1,120 ms. Its audio conversion targets are m4a, mp3, wav, flac and ogg, and compression adds aac and opus on top.

The encode itself is not the differentiator, and it would be misleading to imply otherwise. Smol’s MP3 stream measured 70,634 bps on the alert suite and 105,361 on the tone bed — the same numbers, to the bit, as ffmpeg at -q:a 2. You are not buying a better encoder.

Text metadata survives. Every output carried title, artist, album, album artist, genre and track number, including the 3/12 track position, which is the field batch converters most often mangle.

Embedded cover art does not survive. Measured across MP3, M4A and FLAC output: the text tags came through and the 37,970-byte cover was gone in all three. If album art matters to you, the -c:v copy ffmpeg line above is the better tool today, and we would rather tell you that than have you find out on 400 files.

If you need a specific bitrate rather than whatever the encoder decides, that is a compression setting rather than a conversion one: the range is 16–512 kbps and 8–192 kHz. Asking for 192 kbps produced a file measuring 192,533 bps, which is exactly what was asked for. It also came out slightly larger than the source, because 192 kbps MP3 is a higher rate than the AAC it came from. Converting up is always a waste.

One input limit to know: AIFF is not accepted. Handed one of the/System/Library/Sounds files directly, Smol returns “This file couldn’t be processed. It may be corrupted or in an unsupported format.” Convert AIFF to WAV with afconvert first, or use ffmpeg.

Often not. M4A holding AAC is a better codec than MP3 at the same bitrate, and every Apple device, Android phone, browser and modern car stereo plays it. Converting M4A to MP3 is a lossy-to-lossy transcode: you decode a compressed signal and re-compress the result, losing a little more each time.

There are three reasons that still justify it. An old device or DJ controller that only reads MP3. Software that hardcodes the extension. And a service that will not accept an upload unless it ends in .mp3. All three are real. “MP3 is more universal” in 2026 is not.

If the goal is a smaller file rather than a different container, compress the M4A and keep it an M4A. That is a shorter path with less damage. The same distinction is the whole argument in compressing video on a Mac and in compressing a PDF on a Mac: converting a file and shrinking a file are different operations, and reaching for the wrong one usually costs quality without buying size.

For one file, the Music app. It is free, it is installed, Apple documents it, and it writes the tags and artwork your library already holds. Buying software to convert one M4A is not a close call.

For anything where album art matters, ffmpeg with -c:v copy wins outright and we have shown the measurement that proves it. It is free, it is scriptable, and it is the correct recommendation today.

For a shell script on a machine you do not want to install anything on, afconvert is already there. Just not for MP3, and not if you care about metadata.

What is left for us is narrow and worth stating plainly: a folder of files, once, with consistent settings and no command to remember, handled alongside the images and PDFs you were already compressing. That is the case batch converting audio on a Mac goes into, and if it does not describe you, the free tools on this page are better. If it does, Smol is $29 once and runs entirely on your Mac.