Convert MP4 to MKV

Matroska has been an open container since 2002 and it was designed with almost no opinion about its contents. It will carry H.264, HEVC, AV1, VP9 and FLAC side by side, up to 127 audio tracks, soft subtitles and chapter marks, all in one file. MP4 is stricter: it was standardised as ISO/IEC 14496-14 for delivery, and it handles several of those things awkwardly or not at all.

That is why media libraries drift towards MKV. One file per film that already contains the second language, the commentary and the subtitle tracks is easier to shelve than a folder of companions, and Plex, Jellyfin, Kodi and VLC all read it without being asked twice.

Because the two containers can hold the same H.264 stream, the obvious way to do this is to lift the streams out of the MP4 and write them into a Matroska box untouched. That takes seconds, costs nothing in quality, and is what MKVToolNix — the standard desktop tool for exactly this — does.

This converter does not do that. It decodes the video and encodes it again, which means one generation of quality is spent and a long file takes real time. It is stated here in the second section rather than in a footnote because it changes the recommendation: if the file is a film you intend to keep, remux it properly with MKVToolNix and use this for the cases where a re-encode was going to happen anyway.

This is the part that goes the other way, and it is worth knowing because most converters quietly drop it. All audio tracks are carried, not just the first: an MP4 holding an English track, a German dub and a director’s commentary produces an MKV holding all three, in the same order, each still selectable in the player.

It is the exception on this site rather than the rule. Pulling the same MP4 into an MP3, an AAC or a FLAC keeps only the first soundtrack, because those formats have room for exactly one. Matroska has room for 127, so nothing has to be thrown away.

Matroska’s reputation rests on soft subtitles and chapter marks — text tracks you can switch on and off rather than burn into the picture. This conversion reads the video and audio tracks and nothing else, so an MP4 carrying embedded subtitles produces an MKV carrying none, and chapter marks are lost in the same silence.

Two workable answers. Keep a sidecar SRT file beside the MKV with the same base name, which every media server picks up automatically and which is easier to correct later than an embedded track. Or do the whole job in MKVToolNix, which can mux the video, the audio and the subtitle tracks together in one pass without touching a single frame.

The codec inside the MKV is chosen, not inherited. Whatever the MP4 held, the output is H.264 at High profile with AAC audio — the combination that every media player and every hardware decoder made in the last twenty years handles without help.

For an ordinary H.264 MP4 that is a like-for-like re-encode and the size lands close to where it started. For a 4K HEVC file it is usually a bad trade: H.264 needs more bits than HEVC for the same picture, so the MKV comes out larger while carrying slightly less detail. A modern HEVC file bound for a modern player is one to leave alone.

Quality has three bands and defaults to balanced. Since the video is being encoded anyway, the high band is worth its size for anything you intend to keep, because this is the copy everything downstream will be made from. The small setting is for a file you are about to watch once on a phone.

Resolution only ever goes down. Asking for more than the source has is ignored, deliberately — upscaling invents nothing and produces a bigger file with no new detail in it. Reducing a 4K capture to 1080p once, here, is better than asking a media server to scale every frame on every playback.

No browser plays Matroska. That is not a gap that is about to close and it is the single practical reason to keep an MP4 around: a file you might want to put on a web page, send through a messaging app, or open on somebody else’s phone should stay an MP4. Apple’s own players are the other reliable refusal.

Matroska is also not built for progressive streaming the way MP4 is, so it is a poor choice for anything served over HTTP to a player that starts before the download finishes. Inside a home network, with a server that transcodes on the fly, none of that matters — which is exactly the situation this conversion exists for.

The work runs on your own machine through the hardware encoder the operating system already uses, so a short clip is quick and a long one is proportional. There is no queue and no upload waiting to happen first, which on any hosted converter is normally the slow part of the whole exercise.

The free tier accepts files up to 100 MB. That is enough for episodes, clips and recordings and is not enough for a feature film, and there is no point pretending otherwise — a library-scale job belongs in a desktop tool that can remux a hundred files overnight without decoding a frame.

The conversion happens inside the browser tab, on your processor, using the decoders your computer already has for playing video. Nothing carrying the file is sent anywhere, which you can confirm by opening the network tab and converting something.

For this pair that is more practical than principled. The files are large, the alternative involves sending several hundred megabytes to a stranger’s server and waiting for it to come back, and the finished MKV is going onto a machine two metres away. Moving it across the internet twice to get there was never a sensible route.