Convert M4A to MP3

Almost nobody wants MP3 for its sound. They want it because something in front of them refuses everything else: a car head unit that indexes a USB stick and skips anything that is not `.mp3`, a 2009 Sansa or iPod nano bought for the gym, a church or school PA with a CD-and-USB deck, a voicemail system, a podcast host whose upload form lists one format. MP3 has been unencumbered by patents since 2017 and is thirty-plus years old, which is exactly why the long tail of hardware speaks it.

AAC is the better codec by a clear margin — it was designed in 1997 as the successor to MP3 and does more with the same number of bits — so this conversion moves to a technically worse format on purpose. That is a legitimate thing to do. It is only a bad thing to do when nobody actually required it, which is worth checking before you convert a library of four hundred files.

Your M4A almost certainly holds AAC, which threw away parts of the recording the ear is least likely to miss and cannot give them back. Converting to MP3 decodes what is left to raw samples and then compresses again with a different algorithm that makes its own decisions about what to discard. The two sets of decisions do not line up, so the second encoder spends bits describing artefacts the first one introduced.

In practice, on a 256 kbps AAC album track re-encoded to 160 kbps MP3, most listeners on most equipment cannot tell. The places it shows up are cymbals and applause, which turn slightly grainy, and the very top of the frequency range. The rule that follows is simple: convert once, from the best copy you have, and keep the M4A. Converting the MP3 to something else later is where this genuinely goes wrong.

M4A is a container, not a codec — the registry lists both AAC and ALAC for it — and an `.m4a` file that came off a CD rip in iTunes or Music may well hold Apple Lossless. If it does, everything above is wrong for you: this is a first lossy encode of untouched audio, not a second, and it deserves the High quality setting.

The quickest test is size. Four minutes of AAC is three to five megabytes; four minutes of ALAC is twenty to thirty. On a Mac, Command-I on the file names the codec outright, and iTunes will show "Apple Lossless audio file" in the Kind column if you add it. This converter cannot decode ALAC at all, so an Apple Lossless file stops with a message rather than a bad MP3 — which is an inconvenience and also the answer to your question.

The Quality control offers three bands rather than a number, and they resolve to real MP3 bitrates: Small file is 96 kbps, Balanced is 160 kbps, and High quality is 320 kbps, which is the MP3 ceiling. A four-minute track lands near 2.9 MB, 4.8 MB and 9.6 MB respectively.

For a Voice Memo, a lecture recording or a podcast episode, Small file at 96 kbps is genuinely enough and makes a two-hour recording about 86 MB instead of 288. For music going onto a USB stick for the car, Balanced is the setting. Pick High quality only when the MP3 is an intermediate that something else will process — 320 kbps does not restore anything, it just stops the second encoder from adding a third layer of its own damage.

The registry records MP3 as a two-channel format and M4A as carrying up to 48. An M4A extracted from a film, a concert release or a Dolby mix can therefore hold a 5.1 layout that MP3 has no way to represent.

What happens is not a failure. When no encoder will accept the source layout, the conversion falls back to two channels at 48,000 Hz — so the surround mix is folded down to stereo and resampled on the way. For a soundtrack you want to listen to in a car that is the correct outcome. If the six channels are the point, this is the wrong conversion and there is no setting that fixes it.

An M4A stores its metadata in MP4 atoms and an MP3 stores it in ID3v2, so the tags have to be translated rather than copied. Title, artist, album, album artist, track and disc numbers, genre, date, comment and lyrics are all mapped across, and embedded artwork is rewritten as an ID3v2 APIC frame, which is what every player and phone reads to show a cover.

One category does not survive: anything Apple wrote into the file that has no ID3 equivalent — the raw atoms, purchase records, Apple-specific keys — is dropped rather than transliterated into junk frames. That is deliberate and it is the right call, but if your library depends on a custom iTunes field, check one converted file before you convert the rest.

AAC encoders write the encoder delay and the trailing padding into the file so that a player can skip exactly the right number of samples and run one track into the next with no silence. That is why a live album or a beat-matched DJ mix plays seamlessly in Apple Music.

The MP3 written here carries a Xing header with the frame count, the file size and a seek table, which is what makes seeking accurate in a variable-bitrate file — but it does not carry the delay and padding fields that gapless playback needs. On a continuous album you will hear a short gap at each track boundary. If that matters, convert the whole album as one file, or keep the M4A for listening and use the MP3 only where it is required.

No browser on earth can encode MP3 natively — it is the most requested audio target on the internet and the one thing WebCodecs uniformly refuses — so this page loads a WebAssembly build of the LAME encoder the moment you pick MP3 as the target, and nothing else. The decode side uses your device's own AAC decoder, the same one that plays the file when you double-click it.

Both halves run in the tab, which means the file is never uploaded. You can check that rather than take it on faith: open the network tab of your developer tools and convert something. You will see the page, the analytics and the advertising that pays for it, and no request carrying your audio. For a recorded meeting or an interview under embargo, that difference is the whole reason to use a converter that works this way.

Drop the whole folder. Each M4A converts on its own and the MP3s come back together as a ZIP, and because nothing is uploaded there is no queue and no daily allowance being spent. Files up to 100 MB are handled in the browser, which covers any music track and most long-form recordings, and one drop takes up to 100 files at once.

Two practical notes for a large batch. The conversion is real work — decode and re-encode, per file — so a folder of four hundred tracks will keep a laptop busy for a while and is best done in chunks of fifty. And check the first converted file for its tags and cover art before you commit to the rest; that one look costs a minute and has saved people an evening.