Compress M4A and AAC audio in your browser
This AAC compressor makes M4A, M4B and AAC files smaller by re-encoding them at a lower bitrate. Choose a bitrate, or type the size a file has to fit — an email limit, an upload form, a storage budget for an audiobook — and each file gets the highest bitrate that fits its length. Mono and lower sample rates help speech, and the iTunes-style tags and cover art travel with the audio.
AAC is the format of iPhone voice memos, podcasts, audiobooks, music downloads and most video soundtracks, and it sounds better than MP3 at the same bitrate. Encoding is done on your device by FFmpeg’s AAC encoder (LGPL) compiled to WebAssembly, driven by Mediabunny (MPL-2.0), so files are never uploaded and any bitrate from 16 to 256 kbps is available — not only the 96–192 kbps that the browser’s own encoder accepts on Windows.
How to use the AAC / M4A Compressor
- Drop one or more M4A, M4B, AAC or MP4 audio files on the box. Each row shows the container, codec, length, sample rate, channels, bitrate and size, and whether tags and cover art are present.
- Choose Bitrate and a value, or Target file size and the megabytes per file.
- Pick a Container: keep each file’s own, or write M4A or raw ADTS .aac. Set mono and a sample rate if you want them.
- Press Compress and follow the progress bar; Cancel stops at any time.
- Compare the original and compressed players, then download the files one by one or as a ZIP.
AAC / M4A Compressor features
Any AAC bitrate
AAC-LC from 16 to 256 kbps, also below 96 kbps where speech and audiobooks shrink most.
Target size with a second pass
AAC is encoded to an average bitrate, so if the first result is over the target it is encoded again with a corrected bitrate.
M4A or ADTS
Keep each file’s container or convert: M4A for phones, iTunes and car stereos; ADTS .aac for streaming and broadcast tools.
Tags and cover art
Title, artist, album, track and disc numbers, genre, lyrics, comments and the cover image are copied; between M4A files the other iTunes atoms come along too.
Honest results
Files already at or below your bitrate are kept, and a result that would be bigger than the original is flagged so you keep the original.
Which AAC bitrate do I need?
Size per minute is bitrate × 60 ÷ 8, plus a little for the container and cover art; the audio file size calculator works it out for any length. AAC generally sounds better than MP3 at the same bitrate, most clearly below 128 kbps, so an M4A can often use a lower bitrate than the MP3 you would otherwise make.
| Bitrate | Size per minute | Good for |
|---|---|---|
| 32 kbps mono | 0.24 MB | Speech, voice memos, dictation |
| 48–64 kbps | 0.36–0.48 MB | Podcasts and audiobooks |
| 96 kbps | 0.72 MB | Music on phones and smart speakers |
| 128 kbps | 0.96 MB | Good music quality |
| 256 kbps | 1.92 MB | Typical of purchased music downloads |
AAC-LC, HE-AAC and what this tool writes
Plain AAC (Low Complexity, AAC-LC) is what every device plays. HE-AAC adds spectral band replication, which helps below about 64 kbps stereo. No browser encoder offers it: FFmpeg’s built-in AAC encoder writes AAC-LC only, and in our tests Chrome accepted the HE-AAC profile only at 96 kbps and above and still wrote AAC-LC. So the output is always AAC-LC, and for very low bitrates the Auto sample rate drops to 32, 24 or 16 kHz, which lets the encoder spend its bits on the frequencies that matter.
Re-encoding AAC to AAC is lossy: the decoder’s output is compressed again and a little more detail goes. Halving a 256 kbps download is rarely audible; squeezing 128 kbps music to 48 kbps is. Speech tolerates much more, especially in mono.
Files that can’t be compressed
- Purchased tracks protected with DRM (M4P) cannot be decoded by a browser.
- Apple Lossless (ALAC) in M4A is not decoded by browsers here; for lossless audio use FLAC and the FLAC compressor.
- MP4 videos are accepted, but only the audio is kept; to shrink a video with its picture use the video compressor.
AAC / M4A Compressor FAQ
How do I compress an M4A file?
Drop the M4A on this page, pick a lower bitrate such as 64 kbps for speech or 96–128 kbps for music, or set a target size in MB, and press Compress. Check the result in the player and download it.
Is this AAC compressor free and private?
Yes. It is free, with no sign-up or watermark, and the audio never leaves your device; encoding runs in your browser.
Can I compress an audiobook (M4B)?
Yes. M4B is an M4A with chapter marks. Speech compresses well: mono at 32–48 kbps keeps it clear. Chapter markers are not copied into the new file.
Will the cover art and tags stay?
Yes, with Keep tags and cover art ticked. The cover is embedded once, and between M4A files other iTunes atoms are copied too; untick the box to strip them all.
Why is my file marked “Kept”?
It is already at or below the bitrate you picked with the same sample rate and channels, so re-encoding could only lower the quality without saving space.
Should I choose M4A or AAC (ADTS)?
M4A for almost everything: it plays on phones, computers and car stereos and holds tags and cover art well. Raw ADTS .aac is mainly for streaming servers and broadcast software.