Convert MKV to MP3
Extract the soundtrack from an MKV and get an MP3. The picture is discarded rather than converted, which is why a two-gigabyte video becomes a few megabytes of audio in seconds. MKV is worth knowing about here for one reason the other video formats do not share: its audio is very often five or six channels of AC-3 or DTS rather than ordinary stereo, and turning that into an MP3 means mixing it down.
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Up to 50 MB. Deleted automatically after 30 minutes.
What it does
- The video stream is discarded, not re-encoded
- Multichannel audio is mixed down to stereo
- Finishes in seconds even on a large file
- No watermark and no sign-up
How to use MKV to MP3
- 1
Add the MKV
Drop the file in or choose it from your computer. The 50 MB limit applies to the upload, so long videos may need trimming first.
- 2
Convert
The video is dropped and the audio is re-encoded to MP3. Multichannel tracks are mixed to stereo automatically.
- 3
Take the audio with you
The MP3 plays on any phone, car stereo or player, none of which would have opened the MKV.
Why MKV is the interesting one
Every video format can have its audio extracted, and most of them make it uneventful. MKV is different because of what it was built for.
Matroska is a container designed to hold almost anything, and it became the format of choice for high-quality video precisely because it does not restrict what goes inside it. A single MKV can carry several video tracks, a dozen audio tracks in different languages, subtitles as real text rather than burned into the picture, and chapter markers. Audio in an MKV is very often AC-3 or DTS at five or six channels rather than plain stereo.
That flexibility is what makes extracting audio from one a decision rather than a mechanical step. There may be more than one track, and the track there is may not be shaped like the file you want out.
How it works
The video stream is discarded — not decoded, not converted, simply not carried across — and the first audio track is re-encoded to MP3 at a high variable-bitrate setting. If that track has more than two channels, it is mixed down to stereo as part of the encode.
Because the picture is skipped entirely, this is fast and the result is small. The size difference is not a compression achievement; it is what happens when you stop storing images.
What a downmix actually costs
Mixing six channels into two is not a loss of fidelity so much as a loss of placement. The information is still there — the sound that was behind you is still in the file — but it is now in the same two channels as everything else.
For a film soundtrack that usually means dialog sits less clearly in front of the music than it did, because a center channel exists mainly to keep those two apart. For music recorded in surround, instruments that were separated by position become separated only by frequency and level. For a lecture or a podcast recorded in stereo to begin with, nothing changes at all.
If the placement matters, keep the MKV. It is the only copy that has it.
Examples
A lecture recording you only need to listen to
A two-hour video is an awkward thing to carry around; the audio alone is a fraction of the size and works on any phone or player without a screen.
A concert recording with surround sound
The interesting case. Five or six channels are folded into two, so anything that lived only in a rear or center channel arrives blended into the stereo mix.
Frequently asked questions
Why is the MP3 so much smaller than the video?
Because the picture is thrown away rather than converted. In a typical video the images account for the overwhelming majority of the bytes and the sound for a small fraction, so extracting the audio is close to free. That is also why this finishes in seconds where converting the same file to another video format would take minutes.
What happens to surround sound?
It is mixed down to two channels. MKV is popular precisely because it carries multichannel AC-3 or DTS tracks, and MP3 is a stereo format, so the extra channels have to go somewhere. The mix is a sensible one, but a sound placed deliberately behind the listener arrives in the middle with everything else, and dialog from a center channel sits alongside the music rather than in front of it.
Which audio track does it take if there are several?
The first one in the file. MKV commonly holds more than one — a second language, a commentary, or a stereo track alongside a surround one — and there is no way to choose between them here. If you need a specific track, a desktop tool such as ffmpeg or MKVToolNix will list them and let you pick.
Is the sound quality lower than the original?
Slightly, because the audio is re-encoded. The MKV's own track was usually already compressed, so this is a second pass of lossy encoding and the loss compounds a little. For speech it is inaudible. For music at high quality it is the reason to keep the original file rather than only the MP3.
Further reading
- The Soundtrack Was a Sixth of the File the Video Came InA lecture, an interview, a podcast recorded on camera — plenty of video is kept for its sound. The picture is almost all of the file, and dropping it is the single largest saving available on any video you only listen to.
- Lossless Audio Costs 40% of the Original and People Still Avoid ItLossless audio has a reputation as the choice you make when storage is no object. The measured ratio suggests otherwise: the same recording that took 5.3 MB as an uncompressed WAV took 2.1 MB as FLAC, bit for bit identical. That is a better result than lossless image compression usually achieves, and on difficult material rather than easy.
- What Each MP3 Bitrate Actually Costs, in BytesAudio bitrate is one of the few compression settings where the size is completely predictable — unlike image quality settings, where the same number produces wildly different files depending on the picture. Here is the same recording at six bitrates plus lossless, and what the predictability means for choosing one.