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Use FFmpeg’s stream-copy mode to place compatible MKV audio and video streams in an MP4 container without re-encoding: ffmpeg -i input.mkv -map 0 -c copy output.mp4. This changes the container, not the encoded streams. It also does not make the file a YouTube Live broadcast: live streaming requires an encoder to send a feed to YouTube.
What remuxing does—and what it does not
MKV and MP4 are containers: formats that package video, audio, subtitles, and other tracks. Remuxing copies encoded streams from one container into another. In FFmpeg, -c copy requests that the streams be copied instead of decoded and encoded again. That avoids a new encode, but it does not change a stream’s codec to one the MP4 container accepts.
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Not every MKV can be copied wholesale into MP4. The MP4 muxer may reject a codec or stream type, and subtitle or metadata handling can differ. An error about an unsupported stream is a compatibility problem to diagnose; it does not mean every track must be re-encoded.
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Remux an MKV to MP4 with FFmpeg
1. Inspect the tracks
Use FFprobe, included with FFmpeg, to see the input’s stream indexes, types, codecs, and any available language tags:
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ffprobe input.mkv
Decide which video, audio, and subtitle tracks you need. A file may contain multiple audio languages, subtitles, or other tracks that you do not want in the output.
2. Try copying all streams
Run the command on a copy of the file or a short representative sample first:
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ffmpeg -i input.mkv -map 0 -c copy output.mp4
-i input.mkvnames the input file.-map 0attempts to include every input stream.-c copycopies streams rather than re-encoding them.output.mp4names the new container file.
The -map 0 option is convenient when all tracks are wanted and supported. FFmpeg’s manual also gives a narrower MKV-to-MP4 stream-copy example using -map 0:1 -c copy; the correct mapping depends on the streams in your input.
3. If a stream is rejected, map only the tracks you need
If FFmpeg reports that a stream cannot be muxed into MP4, use the stream details from FFprobe and try selecting compatible tracks. This example selects the first video stream and the first audio stream if one exists:
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ffmpeg -i input.mkv -map 0:v:0 -map 0:a:0? -c copy output.mp4
The question mark makes the audio selection optional, so the command can still run if there is no audio stream. Add further tracks only after confirming the MP4 muxer accepts them. If you need a stream whose codec or type is not supported for the intended output, stream copy cannot convert it; omit it or transcode it, understanding that transcoding is re-encoding.
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4. Verify the output
When FFmpeg finishes, check that the MP4 exists and plays through its expected duration, with the audio and tracks you intended to keep. This is a verification step, not a guarantee that every player or streaming setup will accept the file. Remuxing may complete faster than a full encode because packets are copied, but time depends on the file and storage; no fixed duration is assured.
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A remux gives you a file. To go live, an encoder or playout application must read or otherwise use the source and send a live feed to YouTube. Remuxing can be a preparation step if that software needs MP4, but an MP4 alone is not a live stream, and YouTube’s documentation does not say every live workflow requires an MP4.
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For a file upload
YouTube’s recommended upload encoding settings concern video files submitted for upload. They specify MP4 as a container and include upload-oriented guidance such as H.264 video and placing the moov atom at the front of the file (Fast Start). These upload recommendations are not the requirements for a live encoder feed.
For a live encoder feed
YouTube’s live encoder settings list RTMP/RTMPS, H.264, H.265 (HEVC), and AV1 video, frame rates up to 60 fps, AAC or MP3 audio, and constant-bitrate encoding. YouTube recommends a two-second keyframe interval and says not to exceed four seconds. Recommended bitrates depend on resolution, frame rate, and codec; check the current table for your specific combination rather than treating one number as universal.
For example, YouTube’s current settings page lists 10 Mbps as the recommended H.264 bitrate for 1080p at 30 fps. That is a recommendation for that setting, not a universal bitrate for every live stream. The settings page can change, so consult it again before configuring a broadcast.
Get the live connection details and test
- In YouTube’s Live Control Room, obtain the stream URL and stream key.
- Enter those details in the encoder or playout software you are using. Treat the stream key like a password; do not publish or share it.
- Configure the encoder for an accepted protocol, codec, audio format, bitrate mode, and keyframe interval. Choose the bitrate from YouTube’s current table for the intended resolution, frame rate, and video codec.
- Test with audio and movement similar to what you plan to broadcast. Monitor stream health and any messages in the live dashboard. YouTube’s guidance is direct: “Make sure to test before you start your live stream.”
Common errors and what to do
- “Could not find tag for codec” or an unsupported-stream error: The MP4 muxer does not accept a stream as mapped. Inspect the tracks with
ffprobe, then map only the compatible video and audio tracks. Transcode only if you need a track that cannot be copied into the target container. - The output has no sound or the wrong language: The chosen audio mapping may not match the track you want. Inspect the stream list and select the intended audio stream rather than assuming the first track is correct.
- Subtitles are missing or the command fails when copying all tracks: The subtitle type may not be supported in the MP4 output as mapped. Try omitting that stream, or determine whether your workflow requires converting it; converting subtitles is not the same as copying them.
- The MP4 plays, but YouTube is not live: A local file is not a live feed. Use encoder or playout software to send the stream URL and key from Live Control Room, then check the live dashboard’s health messages.
- The live feed is unstable or rejected: Confirm the selected protocol, video/audio codecs, constant bitrate configuration, and keyframe interval against YouTube’s current settings. Check the bitrate recommendation for your exact resolution and frame rate.
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