Convert AAC to MP3
Convert raw AAC or .aac audio to MP3 with a bitrate selector and batch queue. Runs on your device, so large files are never rejected.
- Runs in your browser
- Files are never uploaded
- Free, no account
First use downloads the 10 MB video engine from this site. After that it is cached and works offline. It is WebAssembly served by onthisdevice.com — never a third-party CDN — and your file is still processed entirely on your own device.
Raw AAC versus M4A
Both hold the same codec, and the difference is entirely about packaging.
| .aac (raw ADTS) | .m4a (MPEG-4) | |
|---|---|---|
| Structure | A bare sequence of frames, each with its own small header | A proper container with an index, a sample table and metadata atoms |
| Tags | None at all | Title, artist, album, artwork, chapters |
| Seeking | Approximate — players estimate from bitrate | Exact, from the index |
| Typical source | Broadcast and stream captures, HLS segments, embedded devices | iTunes, Voice Memos, phone recorders |
| Robustness | Can be cut anywhere and still mostly play | A truncated file may not open at all |
Both are accepted here. If your file is specifically an Apple export, the M4A to MP3 page covers the Apple-specific ground — Voice Memos, GarageBand, DRM — in more detail.
AAC is the better codec. Why convert down?
There is no quality argument for this conversion, and it is worth being blunt about that. AAC was designed as MP3’s successor and is meaningfully better at every bitrate, especially below 128 kbps. Every reason to convert is a compatibility reason:
- Car head units and factory stereos, particularly anything from before roughly 2015.
- DJ software and controllers with fixed format support.
- Cheap MP3 players, gym equipment, in-flight entertainment, hotel clock radios.
- Voicemail systems, telephony platforms and IVR uploads that specify MP3.
- Older transcription and audio-analysis tools with narrow decoders.
If everything you own reads AAC, converting only makes the file bigger and slightly worse. Keep the original.
Choosing a bitrate for a second-generation encode
Because the source is already lossy, the two encoders’ artefacts stack rather than cancel. AAC and MP3 discard different parts of the signal, so anything AAC kept but MP3 handles poorly — pre-echo on sharp transients, high-frequency detail on cymbals — is where the damage shows.
The practical rule is to encode at or above the source bitrate and not to worry beyond that. 256 kbps is the default here because iTunes-era AAC is typically 256 kbps and most broadcast captures sit between 96 and 192 kbps, so 256 covers nearly everything with room to spare. Drop to 128 kbps only for speech.
What you should never do is chain conversions. AAC to MP3 to AAC to MP3 is four lossy passes, and by the fourth the cymbals sound like frying bacon. Always start again from the earliest copy you still have.
When a raw .aac file will not open anywhere
This is common, and it is usually not the converter’s fault. Raw ADTS streams captured mid-broadcast frequently begin partway through a frame or end abruptly. If FFmpeg reports invalid data here, every other tool will read the same broken bytes and reach the same conclusion. Re-download or re-capture the file rather than shopping for a different converter.
A second failure mode: some files with an .aac extension are actually HE-AAC v2
with spectral band replication, and a very small number of decoders handle the parametric
stereo layer badly. FFmpeg handles it correctly, but the output may be mono where you expected
stereo — that is the source, not the conversion.
Batch conversion, no queue, no cap
Up to fifty files convert one after another with their own progress rows, and the finished set downloads as a ZIP. Nothing is uploaded: the AAC decoder and the LAME MP3 encoder are both compiled into the WebAssembly build of FFmpeg that runs inside this tab. That means no per-day allowance, no maximum file size beyond your device’s memory, and no waiting behind other people’s jobs.
Questions
What is the difference between .aac and .m4a?
They usually hold the same audio. A raw .aac file is a bare ADTS stream with no container, while .m4a wraps AAC in an MPEG-4 container that can also carry tags, chapters and cover art. Raw .aac is common from broadcast captures and stream rippers; both work here.
Why convert AAC to MP3 at all, when AAC is the better codec?
Compatibility, almost always. AAC does sound better than MP3 at the same bitrate, but older car head units, DJ software, cheap MP3 players, some gym equipment and a lot of embedded hardware only read MP3. If everything you own plays AAC, there is no reason to convert.
Does converting twice make it noticeably worse?
Each lossy encode discards a little more, and AAC and MP3 discard slightly different things, so the second pass is not free. Encoding at 256 or 320 kbps keeps the artefacts below what most people can hear. Never chain three or four conversions of the same file.
My .aac file will not play anywhere, including here.
Raw ADTS streams captured mid-broadcast are often truncated or start halfway through a frame. If FFmpeg reports invalid data, the file itself is damaged — re-download it rather than trying another converter, because they will all read the same broken bytes.