MP4 to MP3

No. Your browser decodes the audio and encodes the MP3 itself, so the file never leaves your computer — there is no upload, no queue and nothing stored. This tool has no API for the same reason: the work needs the browser's own codecs, and a server version would mean receiving everybody's video to do a job the machine in front of you can already do.

A real MP3. The audio inside an MP4 is almost always AAC, and the common shortcut is to copy that track out and rename it — which produces a file that fails in car stereos, DJ software and podcast uploaders, the exact places people take these. This decodes the audio and encodes MP3 properly, which is why it takes a few seconds rather than none.

192 kbps is the safe default and what most people want. Use 128 for speech — a lecture or an interview — where it halves the size with nothing audible lost. Use 320 if it is music you will listen to properly, or if something downstream demands it. The estimated size updates as you choose, so the trade is visible before you commit to it.

Anything your browser can play. Video: MP4, MOV from an iPhone, MKV, WebM, AVI in most browsers. Audio too, which makes it an audio converter as much as a video one — m4a to mp3 from a voice memo, wav to mp3 from a recording, FLAC or OGG you want in a format something else will accept. If your browser can play it, this can convert it, and if it cannot, the page says so rather than producing a broken file.

Because the audio has to be decoded in full before it can be encoded, and decoded audio is far larger than the compressed file — about 350 KB per second, so an hour is over a gigabyte held in memory at once. Past twenty minutes a browser tab stops reporting errors and simply disappears, which is worse than a limit. The check happens before anything starts, from the file's own duration.

This is that: a video to mp3 converter that runs where the file already is. To extract audio from video with no install and no account, drop the file in and convert — the browser does the decoding, so there is nothing to download and nothing to sign up for. It works offline too, once the page has loaded.

Slightly, and unavoidably. The audio in a video is already compressed, and MP3 is a second lossy pass over it, so this cannot be better than what went in. At 192 kbps and above the difference is not usually audible on the audio a video carries. If you need no further loss at all, you want the original track copied out rather than converted — a different job, and not this one.

Yes, on a reasonably recent phone. Everything runs in the browser, so the phone does the decoding and encoding, and a long file on an old device will be slow and may run out of memory before it finishes. The twenty-minute cap keeps most of that in bounds, but a five-year-old phone converting twenty minutes is asking a lot of it.