Convert AAC to M4A Online
Wrap raw AAC in an M4A container — no re-encoding, in your browser.
.aac · up to 100 MB
What you can do
AAC to M4A: lossless container remux
No re-encoding
The AAC stream is copied intact. Quality is 100% identical to the original file.
Apple-compatible
M4A is the native format for iTunes, Apple Music, Podcasts, and the iOS ringtone pipeline.
100% private
All processing runs in your browser with FFmpeg.wasm. Your audio never leaves your device.
Instant
Container remux is faster than playback. No waiting, no queues.
How it works
Three steps, no hassle
Upload your AAC file
Drag or select your .aac (ADTS AAC) file. Up to 200 MB, no signup required.
Instant remux
The AAC stream is wrapped into an MPEG-4 Part 14 container right in your browser. No re-encoding: audio quality is identical to the original.
Download your M4A
Get your .m4a file ready for iTunes, Apple Music, Podcasts, or any iOS pipeline.
FAQ
Got questions?
AAC (Advanced Audio Coding, defined in ISO/IEC 13818-7 in 1997) can exist as a raw ADTS (Audio Data Transport Stream) in .aac files, or encapsulated inside a container. M4A is simply an MPEG-4 Part 14 (ISO 14496-14) file containing a single AAC audio track. The difference is container, not codec: the compressed audio bytes are exactly the same. The M4A container adds a 'moov' box with metadata (duration, sample rate, channel count, ID3-equivalent tags), allowing players to show progress, seek to arbitrary positions, and read metadata without decoding the entire stream.
iTunes and Apple Music expect valid MPEG-4 containers (M4A, M4B, M4P, M4V) or AIFF/WAV. Raw ADTS .aac files lack the container structure that QuickTime and CoreAudio need to extract metadata, duration, and enable seeking. Apple defined the M4A extension in 2001 to distinguish MPEG-4 audio from MP4 video. Simply renaming a .aac to .m4a without a proper remux produces a corrupt file; a real remux is required to create the correct atom/box hierarchy.
No. This tool performs a pure container remux: the compressed AAC stream is copied bit-for-bit into the new M4A container with no decode or re-encode step. The result is mathematically identical to the original. Any audio quality you perceive comes from the original AAC encoding, not from this conversion.
Yes. M4A/AAC is one of the two universally accepted formats in podcast distribution (alongside MP3). Apple Podcasts, Spotify, and all major distributors (Anchor, Buzzsprout, Podbean, RSS.com) accept M4A. The advantage over MP3 is compression efficiency: AAC delivers equivalent quality at bitrates 25-35% lower. A podcast episode at 128 kbps AAC sounds comparable to 192 kbps MP3.
M4R (Mobile Ringtone) is technically identical to M4A — only the extension differs. iPhone ringtones require M4R with a maximum duration of 30 seconds. The workflow is: AAC → remux to M4A (this tool) → trim to under 30 seconds with an audio editor → rename from .m4a to .m4r → import into iTunes/Finder. The iPhone verifies valid MPEG-4 structure and duration ≤ 30 s; the .m4r extension is the signal to classify it as a ringtone.
LATM (Low-overhead MPEG-4 Audio Transport Multiplex, defined in ISO 14496-3) is an alternative AAC transport format used mainly in DVB broadcast and IPTV. Most .aac files from voice recorders, YouTube rips, and consumer sources use ADTS. If your file uses LATM, the tool will show a format error; you'll need a prior conversion step with FFmpeg (ffmpeg -i input.aac -c:a aac output_adts.aac). The vast majority of .aac files you'll encounter are ADTS.
Convert AAC to M4A: container remux with zero quality loss
AAC (Advanced Audio Coding) was developed by a consortium — AT&T Bell Labs, Fraunhofer IIS, Dolby Laboratories, and Sony — and standardized in 1997 as ISO/IEC 13818-7 (MPEG-2 AAC) as the successor to MP3. Unlike MP3, which was designed within the computational constraints of the 1980s, AAC was built for encoding efficiency on modern hardware: it uses longer transform windows (1024 samples versus MP3's 576), a more flexible filter bank (up to 49 bands versus 32), and temporal prediction on the residual signal. The practical result is that AAC at 128 kbps is perceptually equivalent to MP3 at 160–192 kbps in multiple blind listening studies, including those published by Fraunhofer IIS in 2001. AAC exists in three main variants: AAC-LC (Low Complexity, the most common, used in iTunes, YouTube, and podcasts), HE-AAC v1 (High Efficiency, adds Spectral Band Replication for low-bitrate efficiency, standardized in 2003), and HE-AAC v2 (adds Parametric Stereo for very low bitrates like 24–48 kbps streaming). Raw .aac files contain the AAC stream in ADTS (Audio Data Transport Stream) format, which consists of 7-byte sync headers followed by audio frames. ADTS allows sequential playback but lacks a table of contents for efficient seeking.
M4A (a file extension introduced by Apple in 2001 for MPEG-4 Audio) is an MPEG-4 Part 14 (ISO 14496-14) container that wraps the same AAC stream inside an atom/box hierarchy. The key technical difference lies in file organization: ADTS is a linear sequence of frames; M4A organizes content into two main atoms — 'mdat' (the compressed audio data) and 'moov' (the metadata atom, which includes the time-to-sample table 'stts', chunk offset table 'stco', channel and sample rate info in 'mp4a', and ID3-equivalent metadata in 'udta' or 'ilst'). This structure lets players determine exact duration, seek to any time position in constant O(1) time by consulting the offset table, and read artist/album/artwork without decoding audio. QuickTime (on macOS) and CoreAudio (the iOS and macOS audio framework) require this container structure to function correctly. Apple Music and iTunes reject raw ADTS AAC because without the 'moov' atom they cannot determine duration or display playback progress.
Convertir.ai runs AAC to M4A conversion using FFmpeg.wasm directly in your browser, without sending any audio bytes to external servers. The technical process is a pure container remux: FFmpeg reads the ADTS AAC file, parses the ADTS sync headers (0xFFF or 0xFFE + 28-bit header) to extract sample rate, channel count, and AAC profile, then builds an MPEG-4 atom tree with 'ftyp' (file type, marks M4A compatibility), 'moov' with time tables calculated from ADTS frame timestamps, and 'mdat' with AAC frames copied bit-for-bit without modification. There is no PCM decode step and no re-encode: the process is equivalent to moving data from one envelope to another. Processing speed is orders of magnitude faster than playback time because no audio decoding occurs. Privacy is absolute: FFmpeg.wasm is a WebAssembly compilation of FFmpeg that runs inside the browser sandbox, with access only to files the user explicitly selects, and no ability to access the network or file system.