Convert MKV to MP4

Matroska was published in 2002 as a container with almost no opinion about its contents. It will hold H.264, HEVC, AV1, VP9 and FLAC side by side, up to 127 audio tracks, soft subtitle tracks and chapter marks, all in one file. That permissiveness is exactly why it is used for disc rips, for archival copies and for anything that has to keep more than one language — and exactly why it is the format a television refuses.

No browser plays Matroska. Nor does an iPhone out of the box, nor most smart TVs, nor the upload form on a social platform, nor PowerPoint. VLC and a media server will play almost anything, which is how the file got as far as your computer without a complaint, and it is also why the refusal comes as a surprise on the fifth device rather than the first.

Two files with the same extension can be completely different objects. An MKV from a browser download tool is usually one VP9 video track and one Opus audio track. An MKV made from a Blu-ray is H.264 or HEVC video, two to six audio tracks in cinema codecs, half a dozen subtitle tracks and a chapter list. A recording from OBS is H.264 with several separate microphone and desktop tracks kept deliberately apart.

The conversion treats all three the same way and they come out very differently, which is why the next three sections exist. If you want to see the inventory first, MediaInfo and VLC’s codec information panel both list every track in a file in a few seconds, and knowing what is in there turns this page from advice into arithmetic.

This is the part most converters quietly get wrong. All audio tracks are carried, not just the first: MP4 has no practical ceiling on the number of audio tracks it can store, so a film holding an original language, a dub and a director’s commentary produces an MP4 holding all three in the same order. A player with a track menu will offer all of them.

It is worth knowing because it is the exception on this site. Pulling the same MKV into an MP3, an AAC, a WAV or a FLAC keeps exactly one soundtrack, because those formats have room for one. If the extra languages are the reason you are keeping the file, a video container is the only target that preserves them.

Before a soundtrack is converted, the browser is asked whether it can decode it. If the answer is no, that track is dropped and the picture carries on without it — which means the conversion succeeds, produces a normal-looking MP4, and the MP4 has no sound. Nothing warns you at the moment it happens.

The codecs that fall into this hole are precisely the ones disc rips use: AC-3 and E-AC-3 from DVD and Blu-ray, DTS in its several varieties, and Dolby TrueHD. Those are cinema audio codecs with their own licensing, and browsers do not ship decoders for them. If your MKV came off a disc, play the first ten seconds of the MP4 before you delete anything. An MKV that came out of a browser, a phone or OBS carries AAC or Opus and converts with sound intact.

Soft subtitles are the thing Matroska is famous for — text tracks you switch on rather than burn into the picture — and this conversion cannot see them at all. The engine reads video tracks and audio tracks; a subtitle track is not something it declines to convert, it is something it never notices. Chapter marks go the same way.

The workable answer is a sidecar file. MKVToolNix will extract every subtitle track out of an MKV in seconds without touching a frame, and an SRT sitting next to the MP4 with the same base name is picked up automatically by VLC, by Plex and by most televisions. It is also easier to correct later than a track welded into the container.

The codec written into the MP4 is chosen rather than inherited. Whatever the MKV held, the output is H.264 at High profile — in codec-string form `avc1.64001f` — with AAC audio, because that pairing is the one every device made in the last twenty years decodes in hardware. An MP4 containing AV1 is a perfectly valid MP4 that a television will refuse, which would be trading one unplayable file for another.

The cost lands on modern sources. A 4K HEVC file re-encoded as H.264 usually comes out larger while carrying slightly less detail, because H.264 needs more bits for the same picture. If the destination is a recent phone or a recent TV that already plays HEVC, converting that particular file is a downgrade — the sensible move is to check whether the device actually refused it before spending the time.

Quality has three bands and starts at balanced. Since every frame is being encoded anyway, the high band costs size and buys detail, and it is worth it if the MP4 is going into an editor or is the copy everything downstream will be made from. The small band is for something you are about to watch once on a phone.

Resolution only ever goes down. The list offers 2160p through 360p alongside keeping the original size, and asking for more than the source has is ignored on purpose — upscaling invents no detail and only makes the file bigger. Dropping a 4K rip to 1080p removes three quarters of the pixels and takes the file size with it, which is often the difference between fitting under an upload limit and not.

The work runs on your own machine through the hardware encoder the operating system already uses, so a two-minute clip is done before you have switched tabs and a forty-minute recording is a coffee. There is no queue and no upload, which on a hosted converter is normally the slowest part of the exercise by a wide margin.

The free tier accepts files up to 100 MB. That covers episodes, talks, clips and screen recordings and does not cover a feature film, and there is no point pretending otherwise. A library-scale job belongs in HandBrake or MKVToolNix on the desktop, where a hundred files can be worked through overnight.

Everything happens inside the browser tab, on your processor, with the decoders your computer already has. Nothing carrying the file is sent anywhere, there is no account, and there is no queue behind other people’s uploads.

For this pair the argument is practical before it is principled. The alternative is pushing several hundred megabytes to a stranger’s server, waiting for a re-encode you cannot see, and pulling it back down again — for a file that is going to end up on a device in the same room. Moving it across the internet twice was never the shorter route.