Remux an MKV into an MP4 with ffmpeg on Windows without re-encoding: what plain -c copy silently…

Guide ID: art_01M7BNYC3PWHAFQ3FYDJ7EA91J

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Guide title

Remux an MKV into an MP4 with ffmpeg on Windows without re-encoding: what plain -c copy silently leaves out, why -map 0 fails and leaves an empty file, and a script that keeps every stream it can and verifies each one

Coverage

For people with an MKV or WebM who need an MP4 without re-encoding. A PowerShell script keeps every video, audio and text subtitle stream (text subtitles become mov_text), lists what it leaves out (attachments, data and image subtitles), carries chapters and tags, writes the index first, and checks each copied stream by packet hash and each subtitle by text. It ran on generated clips with ffmpeg 9.0.2, including six refusal cases; no player, real file or image subtitle was tried.

Example

On an MKV with H.264, AAC, Opus, SRT, a font and two chapters, plain -c copy dropped the Opus track and the subtitle without a message, -map 0 stopped with an error and left an empty file, and -map 0 -map -0:t with -c:s mov_text kept everything except the font.

Useful starting points

• Map the streams you want explicitly; automatic selection keeps one stream per type.
• Take the attachments out with -map -0:t and convert text subtitles with -c:s mov_text.
• Delete the empty file a failed command leaves behind.
• Compare packet hashes of each copied stream in the input and the output; open the MP4 in your own player before you delete the MKV.