MP4 to MP3
Nothing is uploaded. The conversion runs here, on this device.
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What this does
An MP4 is a container: a video track, an audio track, and often subtitles and chapter marks in one wrapper. Pulling the sound out means unwrapping the container, taking the audio track, and re-encoding it as MP3.
MP4 files hold video as well as sound. This pulls the audio track out and leaves the picture behind.
What actually happens to the file
The audio inside an MP4 is nearly always AAC, which is already a lossy format. Re-encoding it to MP3 is a second lossy pass, so the result is slightly further from the original than the MP4 was. At 192 kbps that difference is inaudible on ordinary playback; at 96 kbps it is not. Dropping the video is where the size goes: a 100 MB clip commonly leaves an MP3 of two or three megabytes, because the picture was almost all of it.
Nothing is uploaded
The conversion happens inside this page. Your file is read by the browser, converted here, and saved straight back to your downloads. It is never sent to a server, so there is no upload wait and no size cap beyond what your own device can hold.
Open your browser’s developer tools, switch to the network panel, and convert a file. You will see the converter itself download once, and no upload of your audio at all. That is the whole difference between this and a site that converts on a server, and what that difference actually means is worth a read.
The first conversion is slower
The converter is about 32 MB and downloads the first time you use it, which is the one moment this page is not instant. After that it is cached, and later conversions start immediately, even offline. So do later visits.
Questions
Does this keep the video?
No. It takes the audio track and discards the picture — that is the whole operation. If you want the video too, keep the original MP4 as well; converting does not modify it.
Why is the MP3 so much smaller than the MP4?
Because video is most of an MP4. A minute of 1080p video runs to tens of megabytes; a minute of MP3 at 192 kbps is about 1.4 MB. You are not compressing the file so much as throwing away the part that was large.
Will it work on a long video?
Yes, and this is where converting in the browser has an advantage: there is no upload, so a two-hour file does not have to travel anywhere first. The limit is your device’s memory rather than a policy, so a very long file is more demanding on a phone than on a laptop.