Convert MP3 to M4A

Do you need to convert at all?

Honest answer first: iPhones, iTunes and Apple Music play MP3 perfectly well, so you don't need M4A just to listen on Apple devices. Converting makes sense when:

• an app, website or school system accepts M4A but not MP3,
• a teacher, client or colleague has asked for M4A files specifically,
• you want a whole library in one format, matching songs you already have as AAC,
• you're making an iPhone ringtone, which is AAC in an .m4r file; the ringtone maker does that directly.

If none of those apply, keep the MP3s: they already play everywhere, and converting can only cost a little quality. If one does, convert a copy and keep the MP3s anyway, so you can make a fresh version from them later. Since the originals are never touched, that costs nothing but disk space. A quick test helps too: convert one MP3, check it works where you need it, then do the rest in one batch.

The cost of re-encoding

MP3 and AAC are both lossy, so going from one to the other is a second round of compression. AAC is the more efficient codec, and at the same or a higher bit rate the extra loss is very hard to hear. What doesn't work is shrinking: a 128 kbps AAC made from a 320 kbps MP3 is smaller, but AAC can't rescue detail the lower rate leaves out, so use Balanced or Small only when space really matters.

The M4A format can also hold Apple Lossless, available under More settings as the Sound codec. From an MP3 that makes no sense: the file grows several times over and holds exactly the same sound as the MP3. Keep Apple Lossless for lossless sources like FLAC and WAV.

Spoken MP3s, such as talks, lessons and audiobooks, need far less. Small with Mono under More settings gives 64 kbps AAC, which keeps voices clear at roughly 0.5 MB per minute.

Picking a bit rate

So the M4As come out the same size as the MP3s, or a fifth smaller for 320 kbps files. Asking for more than the MP3 had, such as 256 kbps for a 128 kbps song, only makes the M4A bigger: there is nothing extra to keep. For mono MP3s, such as recorded talks, every rate is halved, down to as little as 64 kbps. To set one rate for the whole batch instead, choose it under More settings, Bit rate; the list runs from 320 down to 64 kbps, though AAC rarely needs more than 256.

Tags, covers and your Apple devices

Octoolo copies each MP3's title, artist and album into the M4A, along with the embedded cover picture, so iTunes and Apple Music show the same artwork and sort the songs the same way. Nothing needs re-tagging after the switch.

To get the M4As onto an iPhone from Windows, add them to iTunes (File, Add Folder to Library) and sync as usual. Octoolo also puts each file's index at its start, which lets playback begin before the whole file has arrived, handy when the songs are streamed from a website or a home server. Remove the old MP3 copies from the iTunes library if you don't want every song listed twice. On the iPhone itself, MP3 and M4A songs sit happily side by side in the same albums, so a mixed library is no problem.