MP4 to MKV

How to do it

1. Download Asteria and open it. It's free and needs no account.
2. Drop your MP4 files onto its window, or press ⌘O (Ctrl O on Windows) to choose them. Drop as many as you like.
3. Type mkvand pick Convert to MKV. MKV container with H.264 video.
4. The first time, Asteria asks to download FFmpeg, a free add-on from the FFmpeg project, under the GPL-3.0-or-later license. It shows the download size before you say yes, and needs no administrator password.
5. Press ⌘↵ (Ctrl ↵ on Windows) to save. The MKV files appear next to the originals, which stay exactly as they were.

What it needs

MP4 to MKV uses FFmpeg, a free program for audio and video. Asteria doesn't bundle it. The first time you need it, Asteria offers to download it, checks the download against a pinned checksum, and keeps it in its own folder, where Settings can remove it again. If FFmpeg is already on your computer, Asteria uses that copy.

How it was tested

A real MP4 file was converted on macOS on Apple silicon and Linux, and the MKV that came out was opened and checked with ffprobe to make sure it is what it says it is.