To convert MKV to MP4 quickly without re-encoding, use FFmpeg’s streamcopy: ffmpeg -i input.mkv -map 0 -c copy output.mp4. This changes the container, not the codecs inside. If the streams are not supported by MP4 or by your target device, copy the compatible streams and convert only the incompatible ones.
Choose remuxing or transcoding
MKV and MP4 are containers: they package video, audio, subtitles and other streams. A file with an .mp4 extension is not necessarily encoded in a format your device supports. Renaming an MKV file to .mp4 does not convert it.
| Method | Use it when | Trade-offs |
|---|---|---|
Remux with streamcopy (-c copy) |
Your target accepts the existing codecs and you only need a different container. | Fast and avoids re-encoding quality loss, but may fail if a stream cannot be muxed into MP4. It does not make codecs more compatible. |
| Transcode one or more streams | A video, audio or subtitle stream is incompatible with the MP4 container or intended player. | Selected streams must be decoded and encoded, which takes longer. Compatible streams can still be copied. |
FFmpeg describes streamcopy as useful for changing the container format without decoding and encoding the streams. FFmpeg documentation
Convert MKV to MP4 without re-encoding
Install FFmpeg and open a terminal or command prompt in the folder containing your MKV file. Replace the filenames below with your actual input and desired output names.
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Run
ffmpeg -i input.mkv -map 0 -c copy output.mp4. -
Wait for FFmpeg to finish and check for errors. The output is a new MP4 file; the original MKV is not changed.
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Open the MP4 in the device or app where you intend to use it. Successful file creation does not guarantee that the player supports the codecs inside.
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-c copy copies the selected streams without decoding or encoding them. -map 0 asks FFmpeg to include every input stream. This can include streams that MP4 cannot accept, so the command may fail; it also does not guarantee every kind of track will be preserved as intended.
Keep specific audio, subtitles and other tracks
Explicit mapping is important when you need particular languages, chapters or subtitles. FFmpeg’s automatic stream selection normally chooses one stream of each acceptable type according to its selection rules. Attachments and data streams are not automatically selected. Specifying -map disables the default mappings, so map the streams you want deliberately. FFmpeg documentation on stream selection and mapping
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Inspect the input with
ffmpeg -i input.mkvand note the listed stream indices, languages and types. -
Map the streams you want in the output. For example,
-map 0:1selects input stream 1; add further-mapoptions for other streams you want. Confirm indices from your own file rather than assuming a fixed order.Rank #4
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Run the conversion, then inspect the output and play the relevant audio and subtitle tracks to confirm they are present and usable.
Subtitles may need conversion
A January 2025 FFmpeg-user mailing-list discussion describes a SubRip subtitle stream causing an MP4 muxing error; a reply suggested -c:s mov_text for that case. It is an example, not a universal fix. Image subtitles and other subtitle formats can require different handling or may not transfer as desired. FFmpeg-user discussion, January 2025
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When streamcopy fails or the MP4 will not play
MP4 muxing reports an unsupported stream
Not every stream can be placed in MP4. Check the error and the input’s stream list, then map only the tracks you need. If a needed stream is incompatible, convert that stream rather than re-encoding everything.
Copy compatible streams and encode an incompatible one
FFmpeg supports combining streamcopy with encoding. For example, its manual illustrates encoding video with libx264 while copying audio. The appropriate encoder depends on the source and the target player’s documented codec support; there is no universally correct setting or guaranteed result. Re-encoding takes longer and is distinct from remuxing. FFmpeg manual
The MP4 was created, but a TV or app rejects it
A technically valid MP4 can still contain a video or audio codec the particular player does not support. A January 2025 FFmpeg-user report describes a TV rejecting an output, with codec support discussed as a likely factor; that report is illustrative, not a rule for every TV. Check the device maker’s supported codecs and compare them with the output streams before choosing what to encode. FFmpeg-user discussion, January 2025
You need HDR, Dolby Vision or every original track
Do not assume a conversion preserves HDR or Dolby Vision metadata, attachments, or every track. Check the actual stream formats, your FFmpeg build, the resulting file and the intended device. A successful remux alone does not establish that all metadata or playback behavior was retained.
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