Convert MKV to MP4

How to use it#

1. Drop an MKV file onto the box above, or click it to choose one.
2. The page says whether the video is copied as it is or converted, and what happens to each sound track. Quality applies only when the video has to be converted.
3. Play the MP4 in the preview, then download it.

Why MKV to MP4 is usually quick#

MKV, short for Matroska, is a container, not a codec: a box holding a video track, one or more sound tracks, and often subtitles. The video inside is compressed with a codec, and in almost every MKV that codec is H.264 or HEVC (also called H.265), which an MP4 holds too. Converting takes the compressed video out of one box and puts it in the other without decoding it, which is called remuxing, so the picture is exactly what the MKV held and an hour of recording takes seconds.

Renaming the file is not enough, because the two containers are laid out in completely different ways. A player that expects an MP4 reads the start of the file, finds Matroska there, and refuses it. Converting writes a real MP4 index around the same video.

An MKV holding VP9, AV1, or VP8 video is the exception. Those are converted to H.264 at the original resolution, which can take about as long as the video plays and loses a little detail, and the Quality setting chooses how much. An MKV ripped from a DVD holds MPEG-2 video, and an older download may hold Xvid or DivX. No browser can read those, so the page turns the file away rather than make an MP4 with no picture. It says which of these will happen as soon as the file is read.

OBS recordings#

OBS Studio and other screen recorders offer MKV because it survives a crash. An MP4 writes its index at the end, so a recording cut off by a crash, a full disk, or a lost power cable cannot be opened at all. An MKV cut off the same way plays up to the moment it stopped. The catch is that editors, phones, and upload forms usually want an MP4. A recording that was cut off converts here too, and the MP4 keeps everything it recorded.

OBS usually records H.264 or HEVC video with AAC sound, all three of which go into the MP4 unchanged, so the result is the same size as the recording and loses nothing. A recording with several sound tracks, such as game audio on one and a microphone on another, keeps every one of them in the MP4.

Sound tracks, surround sound, and subtitles#

An MKV from a film or a series often holds several sound tracks in different languages. Each one is kept, with its language, and a player that offers a choice of tracks lets you switch between them. AAC and MP3 sound is copied as it is, and Opus, Vorbis, FLAC, and uncompressed sound are converted to AAC so that phones and QuickTime play them.

Dolby Digital, Dolby Digital Plus, and DTS, the surround formats films carry, are converted to AAC when your browser can read them. Most browsers cannot, and then the track is copied as it is. A TV or a desktop video player plays it, but a browser or a phone may show the film without sound, and the page says so before you download. No browser reads Dolby TrueHD, the lossless track on many Blu-ray rips, so it is left out, while a Dolby Digital track beside it is kept.

Subtitles, chapters, and attached fonts are not carried over; the MP4 holds the picture and the sound. To keep subtitles with the MP4, save them as an SRT file with the same name as the video, which most desktop players and TVs pick up by themselves.