Use FFmpeg’s -c copy option to remux an MKV into an MP4 without decoding and re-encoding its audio and video:
ffmpeg -i input.mkv -map 0 -c copy output.mp4This changes the container, not the encoded streams. It does not, by itself, send a video to YouTube Live: an encoder or playout system must still deliver a live feed using YouTube’s live-stream settings.
What remuxing does—and what it doesn’t
MKV and MP4 are containers: they package encoded video, audio, subtitles, and other tracks. Remuxing copies compatible encoded streams from one container into another. In FFmpeg, -c copy requests that stream copy, so the audio and video are not re-encoded. It avoids an encoding step, but does not convert an unsupported codec into a supported one.
The command’s -map 0 asks FFmpeg to include every input stream. That may include subtitles or other tracks the MP4 muxer cannot accept. Therefore, not every MKV can be remuxed to MP4 with every track intact. FFmpeg documents MKV-to-MP4 stream-copy examples and notes that copying can fail when a stream type is unsupported by the output format: FFmpeg documentation.
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Remux an MKV to MP4 with FFmpeg
1. Inspect the tracks
Use FFprobe, which is distributed with FFmpeg, to inspect the input:
ffprobe input.mkvReview the stream indexes, types, codecs, and any language tags shown. Decide which video, audio, and subtitle tracks you actually need before mapping them into the output.
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2. Try stream-copying all tracks
For a file whose streams are accepted by the MP4 muxer, run:
ffmpeg -i input.mkv -map 0 -c copy output.mp4Replace input.mkv and output.mp4 with your filenames. Keep the original file until you have verified the output. A short representative file is useful for checking a workflow before processing a larger one.
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3. If a track is rejected, map only the tracks you need
If FFmpeg reports an unsupported stream or codec, select compatible tracks rather than asking it to copy everything. For example, this selects the first video track and the first audio track if one exists:
ffmpeg -i input.mkv -map 0:v:0 -map 0:a:0? -c copy output.mp4The question mark makes the audio mapping optional, so the command can proceed if there is no audio stream. Add other tracks only after checking that the MP4 muxer accepts them. If a needed stream is incompatible with MP4, stream copy cannot make it compatible; you may need to omit it or transcode it. Transcoding is re-encoding, not remuxing.
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4. Check the resulting file
Confirm that the output exists, opens in your intended player or ingest software, has the expected duration, and contains the tracks you intended to keep. Check that audio plays and that subtitles behave as expected if you included them. A remux preserves encoded audio and video data in principle, but track selection, container metadata, and subtitle support can affect what appears in the output.
Prepare an upload file or send a YouTube Live feed?
These are separate workflows. YouTube’s recommended upload encoding settings describe file uploads, including MP4 as a recommended container and upload-oriented guidance such as H.264 video and a front-positioned moov atom (Fast Start). Those file recommendations are not the requirements for sending a live encoder feed.
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For a live broadcast, an encoder or playout system must send a feed to YouTube. A remuxed MP4 is still a file; remuxing it alone does not make it live. Whether remuxing is useful depends on whether your chosen encoder or playout software can ingest the source file or needs a different container. YouTube does not say that every live stream requires an MP4 remux.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Configure and check a YouTube Live encoder
Use YouTube’s current live encoder settings for the resolution, frame rate, and codec you intend to send. The supported options and bitrate recommendations depend on those choices; there is no single bitrate that fits every stream.
- Protocol: YouTube’s guidance lists RTMP and RTMPS.
- Video: H.264, H.265 (HEVC), and AV1 are listed. The guidance supports frame rates up to 60 fps.
- Audio: AAC or MP3 is listed.
- Bitrate mode: YouTube recommends constant bitrate encoding.
- Keyframes: YouTube recommends a two-second interval and says not to exceed four seconds.
- Bitrate: Consult the settings table’s row for your resolution, frame rate, and codec instead of applying a universal figure. For example, the current page recommends 10 Mbps for H.264 at 1080p and 30 fps.
Connect the encoder and test the feed
- In YouTube Studio, open Live Control Room and obtain the stream URL and stream key. Enter them in your encoder as directed by YouTube’s streaming setup guidance.
- Treat the stream key like a password. Do not share it publicly; if it is exposed, manage or replace it through the live-stream controls.
- Before the broadcast, test the encoder using audio and movement similar to the planned stream. YouTube says, “Make sure to test before you start your live stream.”
- During the broadcast, watch Live Control Room’s stream health and messages. If YouTube reports a problem, use its message and the relevant codec, bitrate, or keyframe setting to guide the fix.
Copyright and the right to stream the video
Changing a container does not grant permission to use the video, music, or other material in a file. Before uploading or broadcasting, make sure you have the rights or permissions needed for every included element and follow YouTube’s applicable rules. For a practical pre-stream review, use the copyright safety checklist.
Common FFmpeg and live-stream problems
- “Could not write header,” “codec not currently supported in container,” or a similar muxing error: One or more mapped streams may not be accepted by the MP4 muxer. Inspect the tracks with
ffprobe, map only the needed compatible streams, and retry. Transcode only if the target workflow requires a codec the source does not have. - The output plays, but a subtitle or audio track is missing: Check which tracks were mapped and whether the MP4 output supports them. All-track mapping is an attempt to include streams, not a guarantee that every MKV track can be carried over.
- The MP4 exists, but YouTube is not receiving a live stream: A local file is not an encoder feed. Configure compatible encoder or playout software, enter the Live Control Room stream URL and key, and check stream health.
- YouTube reports stream-health or encoding problems: Compare the encoder’s protocol, video and audio codecs, constant-bitrate mode, resolution-specific bitrate, and keyframe interval with YouTube’s current live settings guidance. Test before the event and monitor Live Control Room.
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