MP4 to MP3 Converter

Pull the audio track out of an MP4 and write it as an MP3, entirely inside your browser. The video never leaves your device, and there is no queue to wait in.

The converter is ffmpeg compiled to WebAssembly. It downloads once (30 MB) when you pick your first file, then stays in your browser cache.

A higher bitrate keeps more detail and costs more bytes. 192 kbps is transparent for most music on ordinary gear, 128 is plenty for speech, and 320 is the practical ceiling for MP3.

Your file is processed by your own browser and never uploaded. The engine itself is a one-time 30 MB download the first time you use it, and after that it comes out of your browser cache.

How to use MP4 to MP3

  1. 1Drop your .mp4 onto the box above, or click it to pick a file. Anything up to 500 MB works.
  2. 2The converter fetches itself once, about 30 MB of WebAssembly, then starts reading the audio track.
  3. 3Play the result in place to check it, then download the MP3. Take several as one .zip if you converted a batch.

This is extraction, not conversion

An MP4 is a container. Inside it sit separate streams: usually one H.264 video track and one AAC audio track, interleaved so a player can read both while streaming. So to convert MP4 to MP3 is really to extract audio from MP4 and re-encode it: find the audio stream, decode those samples, hand them to the MP3 encoder, and discard the video entirely.

That distinction matters because it explains the two things people find surprising. The first is speed: no picture is ever decoded, so the job is bounded by how long the audio is, not by whether the video was 4K. A ten minute 4K clip and a ten minute 480p clip take almost exactly the same time. The second is size. A 400 MB video routinely becomes a 9 MB MP3, and nothing is wrong: the video was almost all of those bytes.

The honest cost

The audio in an MP4 has already been compressed once. Re-encoding it to MP3 throws away a little more, because the MP3 encoder has no way of knowing which detail was already discarded and simply makes its own decisions again. This is called generation loss, and it is the reason a chain of conversions degrades while a single one usually does not.

In practice it is a small price for a format that plays on absolutely everything. Pick 192 kbps and most people will not hear a difference on the gear they actually use. Pick 320 kbps if the file is going somewhere you cannot re-do, and 128 kbps if it is speech, where the extra bitrate buys nothing.

What the tool will not do is pretend. It keeps the source sample rate rather than resampling to 48 kHz, and it never claims to improve anything, because decoding and re-encoding cannot recover detail that is already gone.

Size, speed and the ceiling

Files up to 500 MB are accepted. Above that a browser tab genuinely cannot hold the container in memory, and being told so up front is better than having the tab killed halfway through. If your video is larger, trim the part you need first.

Everything runs locally. The engine is ffmpeg compiled to WebAssembly, fetched once and then cached by your browser, so the second file you convert starts immediately.

Frequently asked questions

Does this reduce the audio quality?
Slightly, and unavoidably. The audio inside an MP4 is almost always AAC, which is already lossy, so writing it as an MP3 is a second lossy pass over material that has been through one. At 192 kbps or higher the difference is very hard to hear on ordinary headphones or a car stereo. At 64 kbps it is obvious.
How long does an hour-long video take?
About a minute on a laptop. Measured on a five-minute 720p file, extraction runs at roughly sixty times real time, so the length of the audio is what costs you rather than the resolution. Nothing is queued behind other people either, which is usually why this ends up quicker than a site that does the work on a server.
It says there is no audio track. Why?
Some MP4 files genuinely have none: screen recordings made with the microphone off, exported animations, and silent clips from editing software. Open the file in a player and check that it has sound before assuming something went wrong here.
Is the artwork or the title carried over?
Titles, artists and album names come across where the MP4 declares them. Embedded cover art does not, because the artwork lives in the file as a video stream and this tool drops the video side entirely.
What happens to my file?
It is read by your own browser and processed there. Nothing is sent to us and nothing is stored. The one thing that does come over the network is the converter itself, once, the first time you use it.

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