MP4 to AAC Converter

The sound in an MP4 video is almost always AAC. This tool takes that audio stream out and saves it as a plain .aac file — or, if you prefer, an .m4a — without re-encoding, so the result is bit-for-bit the original sound and the conversion takes seconds even for long videos. Raw AAC (ADTS) files are what many car stereos, streaming encoders, sound-processing tools and older MP3 players expect. Videos with other sound formats are encoded to AAC. Everything happens in your browser.

How to use this tool

1. Choose one or more MP4 (or MOV, M4V) videos.
2. Pick .aac or .m4a, and keep the original sound or choose a bitrate to re-encode.
3. Optionally enter a start and end time, or another audio track number.
4. Download the audio files.

Formula

File size ≈ audio bitrate (kbps) × 125 × length (seconds) bytes

ADTS is AAC with a 7-byte header in front of every frame (1,024 samples, about 21–23 ms), which lets players start anywhere in the file; it carries no title tags. M4A puts the same AAC frames in an MPEG-4 container with tags and an index. When copying, a cut starts on the nearest AAC frame. Re-encoding uses the AAC-LC encoder of the ffmpeg engine; surround sound stays surround at 384 kbps.

Example

A 12-minute video with 160 kbps AAC sound gives an .aac file of 160 × 125 × 720 = 14,400,000 bytes, about 13.7 MB, plus 7 bytes for each of its roughly 31,000 frames (about 0.2 MB).