Convert MP4 to MKV on Mac (Without Re-encoding)

Remux. MP4 and MKV are both containers, and a container swap does not need to touch the video itself.

brew install ffmpeg
ffmpeg -i movie.mp4 -c copy movie.mkv

-c copy copies every stream as is. In my test, a 109,460 byte MP4 became a 106,079 byte MKV, with the same video and audio codecs (checked with ffprobe). Because nothing is re-encoded, the picture is identical and the job is limited by disk speed, not CPU.

For a folder, loop it:

for f in *.mp4; do ffmpeg -i "$f" -c copy "${f%.mp4}.mkv"; done

1. Drag the MP4 files or a folder into Smol.
2. Choose Convert and set the output to MKV.
3. Run it. The MKV appears beside the original with a suffix, and the MP4 is left alone by default.

Smol re-encodes during conversion. In the same test, the MKV came out at 175,983 bytes, bigger than the 109,460 byte source. That is normal when a small, already-efficient file is re-encoded, and it is why a remux is the better choice when your only goal is the container. Smol is the right pick when you also want to change the codec or resolution in the same pass, or when you have a mixed batch of files. To shrink the result, see how to compress video on a Mac.

MKV (Matroska) is an open container with fewer limits. It holds any number of audio tracks, subtitle tracks and chapters in one file, and accepts codecs that MP4 does not officially list. That is why movie rips, anime and archives tend to use it.

For everyday video, MP4 is the safer choice. It plays everywhere, including QuickTime, iPhones, TVs and web browsers. MKV does not: Apple’s avconvert could not read my MKV file at all, and QuickTime Player is not designed to play it. Use VLC or IINA. If you land on an MKV you cannot play, going back is covered in converting MKV to MP4.

This is the practical reason to bother with MKV. MP4 handles only a narrow set of subtitle formats, while MKV stores SRT subtitles as they are. Remux the video and attach a subtitle file in one command:

ffmpeg -i movie.mp4 -i movie.srt -map 0 -map 1 -c copy -metadata:s:s:0 language=eng movie.mkv

I ran this on the test clip. ffprobe showed three streams in the result: H.264 video, AAC audio and a SubRip subtitle track, and the video was not re-encoded. For a second audio track, such as a commentary or another language, add that file as another -i input and another -map entry in the same way.

Players such as VLC and IINA let you switch tracks from a menu. Smol does not add subtitle or extra audio tracks, so this job belongs to ffmpeg.

Yes, and it is lossless in the same way, as long as the codecs inside are ones MP4 allows (H.264, H.265, AAC). Pick only the video and audio streams, since a SubRip track cannot go into MP4 as is:

ffmpeg -i movie.mkv -map 0:v -map 0:a -c copy movie.mp4

That worked on my MKV with an SRT track: it produced a 110,304 byte MP4 and dropped the subtitles. If the MKV holds a codec MP4 does not support, you will need a real re-encode, which is what our MKV to MP4 guide walks through.

One synthetic 5 second 720p clip, so treat the sizes as a pattern, not a prediction.

For a plain MP4 to MKV swap, ffmpeg’s -c copy is better than Smol: faster, free, and lossless. No macOS app does this natively, since QuickTime, Preview and avconvert do not write MKV. Pay for nothing if the command line does not bother you.

Smol makes sense when you want the Finder to do it, when a folder mixes MP4, MOV and other formats, or when you also want smaller files without uploading anything. It is $29 once for one Mac: see Smol. Compare options in the best file converters for Mac or the best video compressor for Mac.