What is the difference between an AAC file and an M4A?

The audio is the same codec. A .aac file is a bare ADTS stream, frame after frame of AAC with a small header on each, while an M4A wraps the same AAC frames in an MP4 container with an index and tags. This page reads both: the first bytes say which one it is, and the stage prints what it found before anything is decoded.

How do I convert an AAC file to MP3?

Leave From on M4A and To on MP3, press Open audio or drop the .aac file on the stage, choose a bitrate, press Convert and then Download. The MP3 keeps the file's name with the new extension.

Does converting AAC to MP3 lose quality?

Some, because both are lossy: the AAC is decoded and the result is encoded again as MP3. At 192 kbps or above the difference is hard to hear for most material, but keep the original AAC if you may need it later.

Is the AAC file uploaded?

No. The stream is parsed and decoded in this browser tab, and the MP3 is written here and handed straight to your downloads.

AAC to MP3 Converter

Open a .aac file and this page reads its first bytes, finds the ADTS stream of AAC audio, decodes it with your browser's own AAC decoder and encodes an MP3 at the bitrate you choose. The page opens in M4A mode, which takes an M4A too, and nothing is uploaded.

Open an M4A or AAC file

· or drop one anywhere on the stage

Never uploadedNo accountUp to 50 MBUp to 10 minutes

From · opens M4A or AAC

Bitrate

Sample rate

Channels

Download

Ready. Choose an M4A or AAC file, or drop one on the stage.

The codec inside the M4A is inspected and named before any conversion begins. A bare .aac file opens here too: it is the same AAC audio without the MP4 box. The output is a constant bitrate MP3 at the bitrate you choose, which is a lossy encode of what came in.

What the bytes sayread from the first bytes of the file, never from its extension
Open a file and its container is identified before anything is decoded.
Output sizeexact arithmetic for PCM, an estimate for MP3
Open a file to size the output.
Decoderasked of this browser before a conversion starts
A codec with no decoder here is named up front, not halfway through.

The From control is a mode, not a guess: the first bytes of the file are read and checked against it, so a file that was renamed is refused with what it actually is. A FLAC opens in WAV mode, and a MOV or WebM video in MP4 mode. An MP4 has its audio tracks listed before anything is extracted and its video is never decoded. Changing the sample rate resamples by linear interpolation, and changing the channel count averages a stereo pair down or copies a mono channel to both sides.

Two open source parts do the heavy lifting. Containers are parsed and audio is decoded by Mediabunny 1.55.7 under the MPL-2.0 (licence text, the exact package, source). MP3 encoding is lamejs 1.2.7 under the LGPL-3.0 (licence text, the exact package, corresponding source). Both are bundled unmodified at those versions and loaded from this page, never from another service.

Common questions

What is the difference between an AAC file and an M4A?: The audio is the same codec. A .aac file is a bare ADTS stream, frame after frame of AAC with a small header on each, while an M4A wraps the same AAC frames in an MP4 container with an index and tags. This page reads both: the first bytes say which one it is, and the stage prints what it found before anything is decoded.
How do I convert an AAC file to MP3?: Leave From on M4A and To on MP3, press Open audio or drop the .aac file on the stage, choose a bitrate, press Convert and then Download. The MP3 keeps the file's name with the new extension.
Does converting AAC to MP3 lose quality?: Some, because both are lossy: the AAC is decoded and the result is encoded again as MP3. At 192 kbps or above the difference is hard to hear for most material, but keep the original AAC if you may need it later.
Is the AAC file uploaded?: No. The stream is parsed and decoded in this browser tab, and the MP3 is written here and handed straight to your downloads.

Files are decoded and re-encoded in your browser, so nothing is uploaded. Converting cannot restore detail a lossy file already lost, and an MP3 output is a lossy encode of its own. Which formats can be decoded depends on your browser, and this page checks the file's actual bytes and its codec before starting rather than trusting the extension.