What an M4A file is, and why MP3 still matters
M4A is the audio-only version of the MP4 format. Inside it is almost always AAC audio: what iPhone Voice Memos records, what iTunes and Apple Music downloads use, and what many phones and recorders save by default. AAC sounds good at small sizes, but plenty of things still want MP3: older car stereos and MP3 players, some podcast and course platforms, DJ and editing software, and upload forms that list ".mp3" and nothing else.
Both formats are lossy, so converting decodes the AAC and encodes it again as MP3. That can't make the sound better than the original, which is why the default setting here looks at the original's bitrate instead of always choosing the biggest number.
Choosing the bitrate
AAC gets more quality out of each kilobit than MP3, so an MP3 needs a bit more room to keep up. Match the original takes the M4A's bitrate, adds about a quarter and rounds up to the next standard MP3 setting: a 128 kbps M4A becomes a 160 kbps MP3, and a 256 kbps iTunes file becomes 320 kbps. Lossless sources such as WAV and FLAC get 256 kbps. Each result shows the original's codec and bitrate. If you pick something far above what the original needs, it tells you. In our test, 320 kbps doubled the file size compared with 160 kbps, without any extra detail to show for it.
| MP3 bitrate | Size per minute | Good for |
|---|---|---|
| 64 kbps mono | 0.48 MB | Voice memos, lectures, interviews |
| 96 kbps | 0.72 MB | Speech in stereo, audiobooks |
| 128 kbps | 0.96 MB | Music from a 96–112 kbps M4A, background listening |
| 160 kbps | 1.2 MB | Music from a 128 kbps M4A |
| 192 kbps | 1.44 MB | Music from a 160 kbps M4A |
| 256 kbps | 1.92 MB | WAV, FLAC and other lossless sources |
| 320 kbps | 2.4 MB | 256 kbps iTunes and Apple Music downloads |
Title, artist and cover art come along
When a conversion loses the tags, a music library fills up with "Track 01" and grey squares. Here the M4A's title, artist, album, album artist, track number, genre, year, comment, lyrics and cover art are written into the MP3's ID3 tag. Each result lists which fields were copied. We checked the output with an independent tag reader: every field matched, and the 600 × 600 cover image was copied byte for byte, not recompressed.
It starts on the first sample
Every MP3 encoder puts a short run of silence at the start of the file, 1,105 samples (about 25 ms) for the LAME encoder used here. Players can skip it, but only if the file tells them how much to skip, in a small header at the start. When we first measured the raw output of the encoding library, that header was missing: decoded in Chrome, the MP3 started 25 ms late and ran 46 ms longer than the original. The converter now writes the header itself. In the same test the MP3 lined up with the original to the sample, and the duration matched exactly: 20.0156 seconds both ways. That matters for albums that play without gaps, for loops, and for audio you will line up with video.
Voice memos and voice notes
Speech recorded on one microphone is mono, and stays mono here, which halves the size
compared with stereo at the same quality. To shrink a stereo recording of a talk or
interview, choose Mono: in our test that took a file from 412 KB to
211 KB. One thing to know about WhatsApp and Telegram voice notes (.ogg or
.opus): Opus is more efficient than MP3, so the MP3 comes out bigger. A
20-second note grew from 80 KB to 160 KB. Convert them when the destination needs MP3,
not to save space. If you only want part of a recording, cut it first with
Trim audio, which works without re-encoding.
Files that won't convert
Songs bought from the iTunes Store before 2009 may be copy-protected .m4p
files, and those can't be converted by any tool without the original account. A file
that is already an MP3 is reported as such rather than re-encoded, because re-encoding
would only lose quality. Video files work too: the sound is converted and the picture is
left out. To get the audio from a video without re-encoding it, use
Extract audio from video.
Your recordings stay on your device
Voice memos are often private: meetings, interviews, notes to yourself. Everything here runs in your browser, including the MP3 encoder, so nothing is uploaded. There's no account or limit, and you can convert a whole folder at once and download it as a ZIP.