MP4 → MP3

Extract a video's audio track to MP3. Free, no upload. 1 GB max · Up to 1 GB · Processed locally, never sent to a server.

A filmed talk, a recorded lecture, an interview, a video memo: sometimes only the sound matters. Keeping the whole video file takes up space out of all proportion to what you are going to do with it, and above all it stops you listening the way you would to a podcast, walking or driving, screen off. Pulling the audio track out as an MP3 settles both problems at once.

Extraction runs in two distinct stages. The browser decodes the audio track of the file itself, through the decoding function of the Web Audio API, and returns a raw, uncompressed signal. That signal is then converted to 16-bit samples and handed to lamejs, the JavaScript port of the LAME encoder, which produces a constant-bitrate MP3. Three bitrates are offered: 96, 128 and 192 kbit/s.

Nothing is uploaded: a video weighing several hundred megabytes does not have to go up and come back down, and there is no queue to wait in. The trade-off is that everything rests on your machine and on your browser. The formats that can be read are the ones it knows how to decode, and the length you can process depends on the memory available. Those two limits account for almost every failure.

How to use it

  1. Drop the video or audio file The drop zone reads "Drop your file here" and takes MP4, WebM, MOV, M4A, AAC, WAV and OGG files, one at a time. A file that holds only audio follows exactly the same path as a video.
  2. Choose the MP3 bitrate 128 kbit/s suits speech and music alike. 96 kbit/s lightens the result for a spoken recording, 192 kbit/s brings it closer to the source without ever going past it.
  3. Start with "Extract to MP3" The button downloads the encoder first, then decodes the whole audio track into memory before any encoding begins.
  4. Follow the progress The progress line shows the number of seconds of audio processed against the total length. The tab has to stay open: the work happens inside the page itself.
  5. Download the MP3 The file keeps the name of the video, with the exact duration and the bitrate chosen recalled next to the size obtained, beside the "↓ Download" button.

How the sound is extracted, then re-encoded

Decoding is not handled by the site but by your browser, which is handed the complete file; what comes out is a floating-point signal, fully decompressed. That signal is then brought down to 16-bit integers, with values that overshoot clipped back to the limit, and passed to the encoder in blocks of 46,080 samples, with a short pause at regular intervals so the page stays responsive. One consequence is worth knowing: decoding resamples the sound to the output rate of your sound card, often 48 kHz, and it is that rate, not the one written into the video, that the MP3 ends up carrying.

The result is stereo at best. A mono track stays mono. A multichannel soundtrack, a film in 5.1 for instance, is reduced to its first two channels, front left and front right. The centre channel, where dialogue usually sits, is not mixed back in, and voices can end up sounding recessed behind the music and the effects.

Length, memory and the real limits

The limit written into the code is 45 minutes: past that, the conversion is refused before it starts. The note under the drop zone suggests staying nearer 30 minutes, and that advice is worth taking.

The reason lies in the decoding step. The decompressed signal has to sit in memory in full before encoding begins, and one minute of stereo at 44.1 kHz in floating point takes up roughly 21 MB. A 40-minute talk therefore calls for something like 800 MB of RAM, which bears no relation to the size of the file you started from. On a phone, the system may discard the tab before the job finishes.

The files that will not go through

A single message, saying the audio could not be decoded, covers three different situations. The first: the video has no audio track at all, the case of a silent screen recording, a time-lapse, or a clip whose sound was stripped during editing. There is nothing to extract, and the error appears straight away.

The second: the container or the codec is not handled by your browser. QuickTime .mov files are accepted at the drop zone but often fail at decoding; the best-served formats are MP4 and M4A in AAC, along with WebM, WAV and OGG.

Since the decoder belongs to the browser and not to the site, trying again in another browser is a serious option. That leaves the third situation: a file locked by a protection scheme, incomplete, or still downloading.

Extracting is not the same as being allowed to

Separating a soundtrack from its picture is a technical operation, and it says nothing about the rights attached to the work. Film music, a track, a podcast, a concert recording all remain protected by copyright, with performers and producers holding related rights of their own. Having the file is therefore not permission to redistribute it, republish it or put it to commercial use.

How much room is left for a copy made for your own use varies a great deal from one country to another: some recognise a private copying exception, others reason through fair dealing or fair use, and several read that room far more narrowly than is commonly assumed. These few lines are not legal advice. If a specific use leaves you in doubt, ask the rights holder or a lawyer. On the technical side, the only network request made is the download of the lamejs encoder, which the site serves from its own files; the video, read from disk, stays on the machine.

Frequently asked questions

My video has no sound, so why the message about decoding?

Because the tool asks the browser to decode an audio track and it finds none. A silent screen capture or a time-lapse therefore triggers exactly the same message as an unsupported format.

Does 192 kbit/s really improve the sound?

It avoids piling loss on top of loss, but it restores nothing. The audio track of a video is already compressed and the MP3 is a second encoding pass. For speech, 128 kbit/s is more than enough and 96 kbit/s stays perfectly listenable.

How big will the file be?

The bitrate is constant, so the arithmetic is direct and independent of the size of the video: roughly 0.9 MB per minute at 128 kbit/s, 1.4 MB at 192 kbit/s and 0.7 MB at 96 kbit/s.

Why does my player show no title and no artist?

The encoder writes the audio stream and nothing else: no ID3 tag is added and nothing is carried over from the metadata of the video. Fill in the title, the artist and the cover art in your player or with a tag editor.

Can I convert a file that is not a video?

Yes. A WAV, an OGG, an M4A or an AAC file is accepted and follows the same route, decoding then re-encoding. That is also the way to shrink a WAV recording, which is very large by nature.

Can I close the tab while it encodes?

No. The work happens inside the page: closing the tab or reloading it stops the operation, and you have to start again from the beginning.

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