Guides / Audio formats, quality and file size
How to convert a Mac recording to M4A or AAC
Tested on macOS 26.4.1 (Tahoe), Apple M3 Max, 3 Sept 2026. Updated 2026-09-03.
You can convert any WAV or AIFF to AAC in an .m4a on a Mac with the built-in command afconvert -f m4af -d aac -b 256000 in.wav out.m4a, no install required. AAC is the codec Apple ships an encoder for; MP3 is not. At 256 kbps the file is about 1.9 MB per minute, and AAC generally sounds better than MP3 at the same bitrate.
What command converts WAV to M4A on a Mac?
Open Terminal and run, tested on macOS 26.4.1:
- Constant bitrate 256 kbps:
afconvert -f m4af -d aac -b 256000 in.wav out.m4a - Variable bitrate, highest codec quality:
afconvert -f m4af -d aac -s 3 -q 127 in.wav out.m4a - A whole folder:
for f in *.wav; do afconvert -f m4af -d aac -b 256000 "$f" "${f%.wav}.m4a"; done
The flags come from afconvert -h: -f m4af is the Apple MPEG-4 audio file container (.m4a), -d aac the data format, -b the total bitrate in bits per second, -q codec quality 0 to 127, and -s the bitrate strategy (0 CBR, 1 ABR, 2 constrained VBR, 3 VBR). AIFF input works the same way. A 10-second 24-bit 48 kHz test file (2,880,044 bytes) came out as a 184,808-byte .m4a with the first command. If you have no Terminal habit, Finder can do the same: right-click the file, choose Encode Selected Audio Files, and pick an AAC quality. For MP3 the story is different, see how to convert AIFF or WAV to MP3.
Is AAC better than MP3?
At the same bitrate, yes. AAC "was designed to be the successor of the MP3 format" and "generally achieves higher sound quality than MP3 at the same bit rate" (Wikipedia, Advanced Audio Coding). AAC files are "typically packaged in an MP4 container most commonly using the filename extension .m4a". Every Apple device, Android, Windows, YouTube and most podcast hosts accept .m4a.
Where MP3 still wins is habit and legacy: some DJ software, car stereos and upload forms accept .mp3 and not .m4a, and 320 kbps MP3 remains the de facto delivery for mixes. So the practical rule is: AAC for listening, podcasts, voice notes and anything that stays on Apple devices; MP3 when a platform demands it. Both are lossy, so neither replaces the PCM master, a point made in WAV vs AIFF vs MP3.
What bitrate should I use for AAC?
Size follows the bitrate: bytes per minute = bitrate / 8 x 60. Anchored on the measured 2.4 MB per minute for 320 kbps MP3, the same arithmetic gives:
- 128 kbps AAC: 0.96 MB per minute, 58 MB per hour. This is ffmpeg's default for its native AAC encoder, whose docs say "If this option is unspecified it is set to 128kbps" (ffmpeg codecs documentation). Fine for speech.
- 192 kbps: 1.44 MB per minute, 86 MB per hour. Music for casual listening.
- 256 kbps: 1.92 MB per minute, 115 MB per hour. A sensible default for music you will keep listening to.
Compared with the 16.5 MB per minute PCM master, 256 kbps AAC is 8.6 times smaller. These are arithmetic figures, not measured on the same 45-file set as the PCM and MP3 numbers in how big is an audio recording per minute; VBR files vary with the material.
What about Apple Lossless (ALAC) instead of AAC?
If you want a smaller file with no loss, use afconvert -f m4af -d alac in.wav out.m4a. ALAC is lossless, so it restores the exact PCM samples, plays in Music and on iPhone, and keeps metadata. The saving depends entirely on the material: a synthetic 10-second sine test shrank from 2,880,044 to 1,169,191 bytes, but a dense DJ mix compresses far less, so no single per-minute figure is honest here. Convert back with afconvert -f WAVE -d LEI24 in.m4a out.wav.
Choose ALAC for archives you want to keep lossless but on Apple devices; choose WAV for anything you will hand to non-Apple software; choose AAC when size matters more than a perfect copy. The -d 0 option copies packets without re-encoding, useful for moving an AAC stream from a video container into .m4a: afconvert -f m4af -d 0 in.mp4 out.m4a, which also covers pulling the audio out of a screen recording, see how to extract audio from a video file on a Mac.
Lossy and lossless options afconvert can write on a Mac
| Format | Lossless | MB per minute | Container / extension | afconvert -d value | Better than MP3 at equal bitrate | Plays in Music and on iPhone |
|---|---|---|---|---|---|---|
| AAC 128 kbps | no | 0.96 (arithmetic) | .m4a | aac -b 128000 | yes | yes |
| AAC 256 kbps | no | 1.92 (arithmetic) | .m4a | aac -b 256000 | yes | yes |
| MP3 320 kbps | no | 2.4 (measured) | .mp3 | cannot write | not applicable | yes |
| ALAC | yes | varies with material | .m4a | alac | not comparable (lossless) | yes |
| WAV 24-bit 48 kHz | yes | 16.5 (measured) | .wav | LEI24 | not comparable (lossless) | yes |
Arithmetic rows use bitrate / 8 x 60; measured rows come from 45 real recordings.
Where AirCheck fits
AirCheck ($39, macOS 14 or newer) does not export AAC; its export options are WAV 16-bit, WAV 24-bit and MP3 320 kbps, chosen before the take and applied once when it stops. Where it fits this workflow is upstream: it gives you a clean 24-bit master of system audio or an interface input, at 16.5 MB per minute, that you then run through the one-line afconvert command above to get a small .m4a for your phone or a podcast host.
When you do not need it: For the conversion itself you need nothing but Terminal; afconvert is already on your Mac. If you want AAC straight out of the recorder, QuickTime Player's audio recording writes .m4a directly (microphone only).
AirCheck records what your Mac plays, or any input, to one file. $39 once.
macOS 14 or newer. One-off payment. Fourteen-day refund, no questions asked.
Questions people ask
How do I convert WAV to M4A on Mac without software?
Open Terminal and run afconvert -f m4af -d aac -b 256000 in.wav out.m4a. afconvert is part of macOS. Finder's right-click Encode Selected Audio Files does the same with a menu.
Is M4A the same as AAC?
M4A is the container (an MP4 file holding audio only); AAC is the usual codec inside it. An .m4a can also hold Apple Lossless. afconvert picks the codec with -d aac or -d alac.
What bitrate is good for AAC?
256 kbps for music (about 1.9 MB per minute), 128 kbps for speech (about 0.96 MB per minute). ffmpeg defaults to 128 kbps. Both are far smaller than the 16.5 MB per minute PCM master.
Can afconvert convert M4A back to WAV?
Yes: afconvert -f WAVE -d LEI24 in.m4a out.wav. From AAC the result is a decoded copy of a lossy file, not the original quality; from ALAC it is bit-identical to the source. Either way the WAV is 16.5 MB per minute at 24-bit 48 kHz.
Sources
- ss64: afconvert man page (-f, -d, -b, -q, -s options)
- Wikipedia: Advanced Audio Coding (successor to MP3, .m4a container)
- FFmpeg: codecs documentation, native AAC encoder defaults
- AirCheck: measured PCM and MP3 sizes (first-party)
Related guides
- How to convert AIFF or WAV to MP3 on a Mac
- WAV vs AIFF vs MP3: which format should you keep on a Mac?
- How big is an audio recording per minute on a Mac?
- How to extract audio from a video file on a Mac
- 16-bit vs 24-bit audio recording: which should you choose?
- 44.1 kHz vs 48 kHz: which sample rate should you record at?
- Audio formats, quality and file size: all guides
- Every guide, by topic