Convert MP4 to MKV

Why move an MP4 into MKV

MKV, or Matroska, is an open container that holds any video and sound codec, many sound and subtitle tracks, and chapters. People move MP4s into it for a few reasons:

• A media server or player library, such as Plex, Jellyfin or Kodi, where everything else is already MKV.
• Adding tracks later. Tools like MKVToolNix can add a commentary, another language or subtitle files to an MKV without re-encoding.
• A program that expects MKV, or simply a habit of keeping an archive in one format.

MKV is not the format to share, though. iPhones, many TVs, video editors and upload forms do not take it, so keep the MP4 for those. Nothing about the conversion touches the original, so you can have both, sitting side by side in the same folder.

What the copy keeps

MKV accepts every codec there is, so with As it is nothing in an MP4 ever needs converting. The H.264, HEVC or AV1 picture goes across untouched, and so does every sound track, whether it is AAC, MP3 or AC-3. The new file is about the size of the old one, and it takes about as long as your disk needs to write it. No graphics card or processor power goes into it, which is why several files copy side by side.

In the rare case that a copy fails, for example with a damaged MP4, Octoolo tries again by converting, and the line then names the codec it is using instead of saying the file was copied. Either way, the picture keeps the size and frame rate the MP4 had.

When converting is the point

Choose High, Balanced or Small and the MKV is encoded afresh. That is how you shrink a library for a media server: under More settings, MKV offers H.264, H.265 (HEVC), AV1, VP9 and MPEG-4 for the picture, and AAC, Opus, FLAC, MP3, AC-3 or Vorbis for the sound. HEVC at Balanced makes much smaller files, which media servers and newer TVs generally handle. FLAC sound is there for an archive that must keep every bit of a recording's sound.

Encoding runs on the graphics card when it can, one video at a time, and Size can bring 4K down to 1080p on the way. For plain shrinking, though, the video compressor keeps MP4, which plays in more places. And the trip back, MKV to MP4, is just as clean as this one.

A library in one batch

This conversion is at its most useful for a whole collection. Drop the top folder: Octoolo goes three levels into it, so season or year folders come along, and lists up to 500 MP4s at a time. Since nothing is encoded, four copies run side by side, and the closing summary puts the total size before next to the size after, which for MP4 to MKV is close to no change at all.

Each MKV is saved beside its MP4 with the same name. If the MP4s sit on a drive Octoolo cannot write to, the MKVs are saved in Videos, in a folder called Octoolo, and the file's line says where. Nothing is ever replaced: an existing film.mkv makes the new one film (2).mkv. Each MKV is a full copy, so the drive needs about as much spare room as the MP4s already take; check that before starting on a large library. Once you have checked a few MKVs in your player, the MP4s are yours to keep or remove.