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Compress AAC audio

Re-encode AAC audio at a lower bitrate or in mono, right in your browser. Nothing is uploaded.

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AAC · MP3 · WAV · FLAC · M4A · AIFF · WMA

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How to compress AAC files

  1. Add your files

    Drop files on the box, paste them or choose them from your device. Nothing is uploaded.

  2. Check the settings

    The default preset suits most files. Change quality, size or format if you need a smaller file.

  3. Download the result

    Save each file or all of them as one ZIP. If a file cannot get smaller, you keep the original.

AAC, the successor to MP3

AAC (Advanced Audio Coding) was standardized in 1997 as part of MPEG-2 and later MPEG-4, by many of the same organizations that created MP3. It fixes several MP3 weaknesses: a more flexible transform, better handling of sharp attacks such as drums and castanets, and support for sample rates up to 96 kHz and up to 48 channels. At the same bitrate AAC generally sounds better than MP3, which is why it became the audio format of iTunes and Apple Music, YouTube, most streaming services, digital TV and practically every video file in MP4.

An AAC stream can be stored in two ways. A bare .aac file is a raw ADTS stream, a sequence of small frames each with its own header, used by some recorders, radio streams and Android apps. Inside an MP4 container the same audio is called M4A, which adds tags, cover art and fast seeking.

AAC profiles and bitrates

ProfileTypical bitratesUsed for
AAC-LC (Low Complexity)96–256 kbps stereoMusic, video soundtracks, almost everything
HE-AAC v1 (adds spectral band replication)32–64 kbpsInternet radio, podcasts on mobile networks
HE-AAC v2 (adds parametric stereo)16–32 kbpsVery low bandwidth streams

HE-AAC rebuilds the high frequencies from a small amount of side information instead of storing them, which works surprisingly well at low bitrates but sounds artificial at higher ones. AAC-LC is the safe default and the only AAC profile the tool writes, always at 32 kHz or more (44.1 or 48 kHz in Chrome and Edge on Windows).

For orientation: Apple Music and the iTunes Store sell AAC at 256 kbps, a rate at which most people cannot distinguish it from the CD. For speech, 64 kbps mono is clear and needs 480 KB per minute; the AAC encoder in Chrome and Edge on Windows starts at 96 kbps, so choose MP3 or Opus output for speech files this small.

Compressing AAC files

Because AAC is already lossy, compressing it means decoding and encoding again at a lower bitrate or with fewer channels. Practical choices:

  • Voice message or dictation that is too large: mono, 48–64 kbps, with MP3 or Opus as the output format.
  • Music file for a phone with little storage: stereo, 128 kbps. Below that, artifacts become audible on cymbals and applause sooner than with a fresh encode from a lossless source.
  • Audio for a slide show or website: 96 kbps stereo is usually enough under spoken commentary.

The output keeps AAC: an M4A or MP4 source becomes an M4A file, and a raw .aac stream stays a raw .aac stream. Firefox has no AAC encoder, so there the result is an Opus file instead. If the source already has a low bitrate, there is little to gain, and the tool returns your file unchanged rather than a worse-sounding copy of the same size.

Common problems

  • The file will not seek properly. Raw ADTS streams have no index, so some players cannot jump to a position or show the right duration. Converting it to M4A with a desktop program such as VLC fixes this; the compressor keeps a raw stream raw.
  • Windows does not show the length. Same cause; a container with an index solves it.
  • A file is labeled .aac but will not open. Some apps use the extension for files that are actually M4A or even different codecs. The tool checks the content, not the name.

AAC questions

Is AAC better than MP3?

At the same bitrate, generally yes, especially below 128 kbps. At 256 kbps and above, both are transparent for most listeners, and MP3 has the edge in compatibility with very old devices.

Is AAC the same as M4A?

AAC is the codec; M4A is an MP4 file containing audio, almost always AAC. A raw .aac file holds the same kind of audio without the container.

Can I convert AAC to MP3 to make it smaller?

Usually not worth it. Going from one lossy codec to another at a similar size loses quality, and MP3 is less efficient. Lower the AAC bitrate instead.

Does AAC support surround sound?

Yes, the format allows multichannel audio, and it is used for 5.1 soundtracks on TV and in video files. For sharing, stereo is more compatible and smaller.

See also

Compare with MP3, see all formats on the audio compressor page, or read how it works.