What is an MKV file?

MKV is a container: a wrapper that holds streams encoded by something else. It is used for archiving and handing a finished file over.

The extension is .mkv, and the full name is Matroska Video. Both matter less than what the file can hold, which is what the rest of this page is about.

It dates from 2002. The specification is Matroska.

Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

It usually holds H.264, HEVC, AV1, VP9 and FLAC. This is the distinction behind most "why will this not play" questions: two files with the same extension can contain entirely different encodings, and a player that refuses one may be perfectly happy with the other.

It also explains why changing container is often fast and lossless while changing codec is neither. Moving the same stream into a different box rewrites the box; re-encoding rewrites the picture.

MKV can hold several audio tracks and subtitle tracks.

That matters mostly when converting: whatever the target cannot hold is dropped, usually without a warning.

VLC, MKVToolNix and HandBrake read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

Only some browsers read it.

Where support is uneven, serving a second format alongside it is the usual answer — the browser takes the one it understands and nobody is left out.

MKV is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.

The recurring complaints: support outside its home ground is uneven.

None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.

Matroska was designed in 2002 with an explicit goal: put no limits on what can go inside. Any video codec, any audio codec, any number of tracks of each, subtitles, chapters, attachments, cover art. Where MP4 defines a list of things it permits, MKV defines a way to store anything and lets the player worry about it.

That is why a film in MKV can carry the original audio plus two dubs plus a commentary, eight subtitle languages, chapter markers and the fonts the subtitles need — all in one file. Nothing else in common use does that, and it is the reason people who choose a container choose this one.

Matroska is a free, open specification with no licensing body and no patents attached to the container itself. Anyone may implement it, which is why it is the default in open-source tooling and why WebM — the container browsers use for VP9 and AV1 — is a deliberately restricted subset of it.

The codecs inside can still carry their own licensing, and usually do. The distinction matters: the box is free, and H.265 inside the box is not. That is the opposite arrangement from RAR, where the format is proprietary and the contents are just files.

Because hardware players implement a short list of containers, and MKV is frequently not on it. This is not about the video inside — the same H.264 stream in an MP4 would play immediately. The device looks at the container, does not recognise it, and stops there.

Smart televisions, games consoles, older streaming boxes and most car systems behave this way. Computers, phones with a real media player, and media server software do not. It is the single most common reason someone converts an MKV, and the reason the conversion is often much cheaper than expected.

This is the part worth knowing, and it is not what happens here. If the video and audio inside an MKV are already formats the destination accepts — usually H.264 and AAC — MKVToolNix or ffmpeg will lift them into an MP4 without decoding anything, and a two-hour film takes seconds. The converter on this site always re-encodes: it hands the encoder a bitrate, and a requested bitrate is exactly what disqualifies the packet copy.

A re-encode is genuinely unavoidable when something inside is not supported at the far end: H.265 going to an old device, or an audio format the target cannot decode. Where it is avoidable, a desktop remux is both faster and lossless, and it is the honest recommendation for a large film whose streams already fit.

The tracks, mostly, which is the irony of the conversion. MP4 will hold several audio tracks and tag each one with its language — an iTunes film does exactly that — but converters write one, so the alternate languages and the commentary are dropped, and the tool picks the first rather than asking. That is how people end up with a film in a language they do not speak.

Subtitles fare worst. MKV holds them as proper selectable tracks; MP4’s timed text is inconsistently supported and frequently ignored by exactly the televisions the conversion was done for. The two honest options are a separate SRT file beside the video, or burning the text permanently into the picture.

MKV supports two kinds. SRT is plain timed text, simple and universally understood. ASS is a styling format with positioning, colour and typography, used heavily for subtitled animation and for anything where captions have to sit around on-screen text.

ASS subtitles reference fonts by name, so MKV lets the fonts themselves be attached inside the file. That is why some MKVs are inexplicably larger than their video accounts for, and why extracting the subtitles alone can produce something that renders in the wrong typeface. There is no equivalent in any other common container.

MKV stores chapter marks natively, so a player can skip between scenes, episodes or sections. Media server software reads them; this is the basis of the skip-intro behaviour people are used to from streaming services.

MP4 has a chapter mechanism too, but support for it is patchy enough that chapters frequently do not survive the trip. For a film this is a minor loss. For a long recording — a lecture, a conference, a set of episodes in one file — it is often the reason to leave the file as MKV and use a player that copes.

Discs ripped to a hard drive, because MKV is the only common container that preserves everything a Blu-ray holds. Media server libraries, where the tracks are the point. Recordings from broadcast tuners. Anything produced by open-source tooling, which defaults to it. And a long tradition of subtitled video distribution, which is where the ASS-plus-attached-fonts arrangement came from.

The common thread is that the file was made to be kept rather than sent. That is a useful framing for what to do with one: an MKV is usually somebody’s library copy, and converting it is about getting it onto a particular screen rather than about improving it.

When the destination is a television or a console, MP4 with H.264 is the answer and the MKV should stay where it is. The converted file is a delivery copy for one device; the original still holds the alternate audio, the subtitle tracks and the chapters, and none of that comes back once discarded.

When the destination is a computer, a phone or a media server, do not convert at all. VLC, MPV, Plex and Jellyfin all handle MKV natively and will use everything in it. Converting to please a device that was never going to see the file is work spent to lose features.