Convert MKV to MP4 on Mac: 3 Methods

OBS captures, stream archives, and some open-source editors save MKV. QuickTime Player and many Mac upload forms expect MP4. Double-click does nothing, Final Cut refuses import, or a portal lists only MP4 and MOV. The fix is usually a container remux or a short re-encode, done locally so unreleased builds stay on your disk.

Why MKV is awkward on Mac

MKV is a flexible container. It can hold H.264, HEVC, VP9, multiple audio tracks, and subtitles. Mac-first apps prefer MP4 or MOV.

If your source is MOV, see MOV to MP4 on Mac . For size after conversion, see how to compress video on Mac .

Method 1: GetCompress

Best for: Folders of captures, preview and trim, and portals that also list a megabyte cap.

1. Drop one MKV or a whole folder into GetCompress .
2. Choose MP4 output.
3. Trim to the seconds you need in the preview.
4. Set a quality level (Original, High, Medium, Balanced, or Low) or resolution, or type a target file size when the form states a limit.
5. Export and confirm playback in QuickTime before you delete the MKV.

Processing stays on your Mac. Save a preset such as “1080p H.264 share” for the next time you convert similar OBS files.

Method 2: Remux with FFmpeg

Best for: Instant container swaps when video and audio already fit MP4.

brew install ffmpeg

ffmpeg -i input.mkv -c copy output.mp4

Batch remux:

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

Play the output in QuickTime before you delete the original. If QuickTime shows black video but VLC plays it, the inner codec still needs re-encoding even though remux succeeded.

Limitation: Remux fails when codecs inside MKV are not MP4-compatible. VLC can convert too, but export profiles are easy to mis-pick.

Method 3: Re-encode when remux fails

Best for: HEVC or VP9 inside MKV that must become H.264 for an upload target.

ffmpeg -i input.mkv -c:v libx264 -crf 22 -preset medium \
 -c:a aac -b:a 128k -movflags +faststart output.mp4

Lower resolution for sharing:

ffmpeg -i input.mkv -vf scale=-2:1080 -c:v libx264 -crf 23 \
 -c:a aac -b:a 128k -movflags +faststart output-1080p.mp4

Trim long captures first. A 30-second bug repro at 1080p beats re-encoding a full-hour MKV.

Limitation: Re-encode takes time and is lossy. Keep the MKV master.

Which method should you use

When remux alone is enough

If -c copy produces a QuickTime-playable MP4 under the limit, stop there.

GetCompress fits when OBS folders arrive often, when you need preview and trim before encode, or when format and size must be fixed together. It does not replace a full editor for multi-track mixes.