How to use the MP4 to MKV converter
- Choose the MP4 files, one or a whole folder.
- Keep Resolution on Keep original so the streams are copied.
- Convert to MKV and download.
Why nothing is re-encoded
MKV accepts H.264, HEVC, VP9 and AV1 video alongside AAC, MP3, Opus or AC-3 sound, which covers what MP4 files normally contain. The packets are moved across as they are, so the MKV holds the same media and differs in size by only a sliver of container overhead.
The one exception is a new resolution: choosing one under More settings rebuilds the picture as H.264 at that height.
Where MKV fits
Matroska copes better with an interrupted write than MP4, can hold many audio and subtitle tracks, and is a natural format for Plex and Jellyfin libraries. It suits an archive or a home media shelf.
For anything you send to other people, keep the MP4. Browsers, PowerPoint, Instagram and iPhones all handle MP4 far more reliably than MKV.
Codecs stay as they were
Because the video track is copied, an MP4 holding HEVC gives you an MKV holding HEVC, and the player still needs to decode HEVC. A container change never makes a clip easier to play on a weak device.
Subtitles are not added here. The MKV gets the MP4’s picture and sound; mux extra subtitle tracks in afterwards with a tool such as MKVToolNix.
Questions
Will my phone open MKV files?
Android usually can. On an iPhone you need a player such as VLC, so MP4 is the safer choice for phones.
Does quality drop?
Not when the streams are copied. Only picking a lower resolution triggers a re-encode.
Is the MKV larger?
Only by a fraction of a percent. Both containers add similar overhead and the media inside is the same.
Can I convert a whole library?
Yes. Drop a folder of MP4s and every one is rewrapped in turn; the ZIP button gathers the MKVs.