Convert MP4 to AAC — Free Online Tool
Extract and convert the audio track from an MP4 video file into a standalone AAC file using FFmpeg's native AAC encoder. AAC delivers better sound quality than MP3 at equivalent bit rates, making it the ideal choice for Apple devices, iTunes, and modern streaming workflows.
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FFmpeg Command
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Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
MP4 files typically store audio as an AAC stream inside a container alongside video and other tracks. This conversion strips away the video, subtitle, and chapter data entirely, then re-encodes (or in many cases near-passthrough encodes) the audio stream as a raw AAC file with an .aac extension. Because the source MP4 likely already uses AAC audio (the default for MP4), the re-encoding step is minimal in quality loss — the audio is decoded from the container and re-encoded at the target bit rate using FFmpeg's built-in AAC encoder. The output is a pure audio file with no container overhead beyond the basic ADTS framing used by raw AAC files.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg tool, which handles all media reading, decoding, encoding, and writing in this conversion pipeline. |
-i input.mp4
|
Specifies the input MP4 file containing the video, audio, and any other tracks that FFmpeg will read and demux for processing. |
-c:a aac
|
Sets the audio encoder to FFmpeg's built-in AAC encoder, which will encode the extracted audio stream into AAC format for the output .aac file. |
-b:a 128k
|
Sets the audio bit rate to 128 kilobits per second, which is the standard default for AAC and provides a good balance between file size and audio quality for music and voice content. |
output.aac
|
Defines the output file as a raw AAC file using ADTS framing. The .aac extension tells FFmpeg to write a bare AAC bitstream rather than wrapping it in an MP4 or M4A container, and no video or subtitle streams are included. |
Common Use Cases
- Extracting a music soundtrack or score from an MP4 video to load into iTunes, Apple Music, or an iPhone without carrying unnecessary video data
- Pulling dialogue or narration audio from an MP4 interview or lecture recording to distribute as a standalone audio file compatible with iOS podcast apps
- Stripping the audio from a screen recording or tutorial video to create a lightweight audio-only version for users who only need the voiceover
- Preparing AAC audio assets from MP4 source footage for use in video editing timelines or DAWs that prefer raw AAC over containerized MP4 audio
- Archiving just the audio commentary track from an MP4 film or event recording when the video is no longer needed but the audio must be preserved
- Reducing file size dramatically for sharing audio content derived from MP4 videos over messaging apps or email where video attachments are too large
Frequently Asked Questions
There will be a small amount of quality loss because the audio is decoded from the MP4 container and re-encoded to a new AAC file rather than being copied byte-for-byte. However, since both the source and destination use the AAC codec, the generational loss is minimal — especially at 128k or higher. If you want zero additional quality loss, you could use '-c:a copy' with an MP4 or M4A output instead, but that requires keeping a container format.
An M4A file is AAC audio wrapped inside an MPEG-4 container (essentially an MP4 with only an audio track), while a raw .aac file uses ADTS framing without a full container. Raw AAC files are slightly simpler and widely supported, but M4A files can carry richer metadata like album art, track titles, and chapter markers. If you need iTunes-style metadata, converting to M4A is preferable; for raw audio extraction or streaming use, the .aac format works perfectly.
Raw AAC files using ADTS framing have very limited support for embedded metadata compared to container formats like M4A or MP3. FFmpeg will attempt to copy over basic tags from the MP4 source, but most metadata — including album art, chapter markers, and extended ID3-style tags — will not survive in a raw .aac output. If preserving rich metadata is important, consider outputting to M4A instead.
Replace the '128k' value after '-b:a' with your desired bit rate. For example, use '-b:a 192k' for higher quality suitable for music, or '-b:a 96k' to reduce file size for voice recordings like podcasts or interviews. AAC performs well even at 96k for speech, and 192k–256k is generally considered transparent (indistinguishable from the original) for music at typical listening conditions.
Yes — AAC is natively supported on Android as well as iOS, Windows, macOS, and most modern browsers. While AAC is closely associated with Apple due to its use in iTunes and the M4A format, it is part of the MPEG-4 standard and has broad hardware and software decoder support across all major platforms. The raw .aac format produced by this command will play on virtually any modern device.
The command shown processes a single file, but you can adapt it for batch processing using a shell loop. On Linux or macOS, run: 'for f in *.mp4; do ffmpeg -i "$f" -c:a aac -b:a 128k "${f%.mp4}.aac"; done'. On Windows Command Prompt, use: 'for %f in (*.mp4) do ffmpeg -i "%f" -c:a aac -b:a 128k "%~nf.aac"'. This will process every MP4 in the current directory and produce a matching .aac file for each.
Technical Notes
The FFmpeg native AAC encoder used in this command ('aac') is a solid general-purpose encoder built into FFmpeg, though the optional 'libfdk_aac' encoder (when compiled in) is considered higher quality, particularly at lower bit rates below 96k. For most use cases at 128k and above, the native encoder produces results that are perceptually equivalent. Raw AAC output uses ADTS (Audio Data Transport Stream) framing, which is a simple byte-stream format without a full container — this is what gives the file its .aac extension. One important limitation is that ADTS AAC does not support multiple audio channels beyond standard stereo in a straightforward way, and multi-track audio from the source MP4 is not preserved — only the default audio stream is extracted. Chapter markers and subtitle tracks from the MP4 are also fully discarded, as the AAC format has no mechanism to carry them. If your source MP4 contains a non-AAC audio codec such as Opus or MP3, the re-encoding step will be more significant and careful attention to bit rate selection is advisable.