How the audio is converted
The video track is discarded and the audio track is decoded and re-encoded as MP3 at the bitrate you choose. That second part is worth stating plainly, because it is the difference between what this does and what a stream copy does. The audio inside an MP4 is almost always AAC, and MP3 is a different codec, so the audio has to be decoded and compressed again rather than simply lifted out.
That has one practical consequence: you are compressing something that was already compressed, so a small amount of quality is lost on the way through. It is not usually audible at a sensible bitrate, but it is real, and it is why picking 320 kbps does not make the result better than the source. It only stops the re-encode from taking more away.
If you want the audio without a second compression pass, use MP4 to M4A instead. That keeps the audio in the AAC family it was already in.
- Load a video and pick a bitrate
- Optionally set a start and end point
- Convert and download the MP3
Choosing a bitrate
Four options are offered: 128, 192, 256, and 320 kbps, with 192 as the default. Higher numbers mean a larger file and less loss in the re-encode.
For speech, a podcast, a lecture, or an interview, 128 is genuinely sufficient and produces a file about 1 MB per minute. For music, 192 is the sensible default and 256 is worth it if the source was high quality to begin with. 320 is the maximum the format allows and mostly buys peace of mind rather than an audible difference.
The one thing to keep in mind is that the source sets the ceiling. If the video audio was encoded at a low bitrate, converting to 320 kbps produces a large file that sounds exactly like the low-bitrate original. There is nothing to recover.
Trimming and file size
A start and end range lets you convert only a section, which is the fastest way to pull a single quote out of a long recording without producing an hour-long MP3 and cutting it afterward. Setting the range also makes the conversion quicker, since only that section is decoded.
Everything runs in browser memory, so the whole video has to fit. Files up to around 1.5 GB convert reliably and larger ones can fail partway through. A long screen recording or a high-bitrate camera file is where you are most likely to hit that, and trimming the source with Video Trimmer first is the reliable answer.
The same page can also produce WAV, M4A, and Ogg output. WAV is uncompressed and much larger but adds no further loss, which makes it the right pick when the audio is heading into an editor for further work.
- Trimming first is faster and produces exactly the clip you need
- Practical ceiling is around 1.5 GB of source video
- WAV output avoids a second lossy compression pass