About AAC to MP3

AAC turns up under several extensions, and all of them work here. A raw .aac stream, an .m4a music track, an .m4b audiobook, an .m4r iPhone ringtone and an audio-only .mp4 are all the same codec in different wrappers, so they all come out as MP3.

You can queue 20 files of up to 100 MB each. Mono stays mono and stereo stays stereo, which keeps voice recordings from doubling in size for no reason; a 5.1 soundtrack is down-mixed to stereo, because MP3 has no practical surround mode. The one thing that cannot work is DRM-protected audio — the old iTunes .m4p format is locked at the codec level and no browser will decode it.

There is no conversion server behind this page. Every file goes through the same three stages, all of them inside your browser tab:

1. Decode. The file is handed to decodeAudioData, the same Web Audio call a browser uses to play AAC back. That gives raw PCM audio at the sample rate you selected — 44.1 kHz by default, 48 kHz if the audio came from video.
2. Encode. The PCM is passed to a LAME encoder — the reference MP3 encoder, compiled to JavaScript — which writes MP3 frames at your chosen constant bitrate. This is the slow part, so it runs in a Web Worker on a background thread and reports progress as it goes.
3. Hand back. The finished frames are assembled into an audio/mpeg blob and wired to a download link. Your browser saves it straight from memory.

Because the decoder is the browser's own, there is no multi-megabyte codec to download before the first conversion starts, and the tool keeps working with the network switched off once the page has loaded.

AAC and MP3 are both lossy codecs, so converting between them re-compresses audio that was already compressed once. No tool can avoid this — the original detail discarded by the AAC encoder is not recoverable, and the MP3 encoder then makes its own cuts.

What you can control is how much room the second pass gets. At 256 and 320 kbps the difference is inaudible to virtually everyone; 192 kbps, the default here, is a sensible balance for music; 128 kbps is fine for speech, where the smaller file matters more than the top octave. If the recording is irreplaceable, export the MP3 for the device that needs it and keep the AAC original as your master.

Nothing you drop here is uploaded. The decode and encode stages both run on your own device, so AAC to MP3 never receives your audio, its filename or its metadata — there is no bucket of other people's recordings for yours to sit in, because there is no bucket.

The audio exists only as memory held by the tab. Remove a file from the list, close the tab or navigate away and it is released; nothing is written to storage on your device, so nothing about a conversion carries over to your next visit.

This covers what the tool itself does. It says nothing about what a browser extension, a corporate proxy or the device itself might see — only that our side of it is empty, which is the part we can actually promise.