Move the video of an MKV into an MP4 without re-encoding it, converting only audio that MP4 players can't handle. Nothing is uploaded, and there’s no account, watermark or limit.
Drop your recordings or films where the page says “Drop MKV files here”, or tap to select them. Each row tells you what will happen, such as “Fast: copied into MP4 without re-encoding” or “Video copied; AC3 audio converted to AAC”.
For an HEVC (H.265) MKV that must play on an older TV or a Windows PC, tick “Maximum compatibility (convert HEVC to H.264)”. Otherwise leave it off so the picture is only copied.
Press “Convert 1 video to MP4”; with several files the button shows how many, and they are handled one by one.
Use Download on each finished row, or “Download all (.zip)” for the whole batch. “Clear list” empties the page for the next set.
MKV is the file extension of Matroska, an open container format started in 2002 and named after the Russian nesting dolls. A single MKV can hold almost any codec and any number of tracks: one video, several audio languages, subtitle tracks, chapters and even attached fonts. That flexibility made it the standard for film and series releases and a popular choice for game and stream recordings.
OBS Studio users meet MKV for a practical reason. If OBS, the PC or the power fails mid-recording, an unfinished MP4 is usually unreadable because its index is written only at the end, while an MKV stays playable up to the moment of the crash. Recording to MKV and converting afterwards is the safe workflow.
The catch is playback. The iPhone Photos app, iMovie, many editing programs, some smart TVs and a lot of upload forms won't accept MKV even though the video inside is ordinary H.264 or HEVC. That is why converting is usually a remux: filetidy copies the video stream as is into an MP4, and only touches the sound when it is a codec MP4 players handle poorly. AC-3, E-AC-3, DTS, FLAC, Opus or PCM sound becomes stereo AAC at 160 kbit/s; AAC or MP3 is copied.
An MP4 made here contains one video track and one audio track: the main ones. Extra audio languages, commentary tracks, subtitles and chapter markers are left behind, so keep the MKV if you need them.
Windows
OBS works the same way on Mac and Linux. In Settings > Advanced > Recording you can also tick “Automatically remux to mp4” so every MKV recording gets an MP4 copy when you stop.
Mac
HandBrake always re-encodes the video, so it is much slower than a remux, but it is the tool to use when you need a second language or burned-in subtitles.
Keep MKV as your archive and for anything played in VLC, Kodi, Plex or another player that reads it: it preserves every language, subtitle and chapter, and it survives crashes during recording. Nothing is gained by converting a file that already plays where you watch it.
Make an MP4 when a specific device or app refuses the MKV: an iPhone or iPad, iMovie or another editor, a TV that only lists MP4 files from USB, or a site that only takes MP4 uploads. Because the video is usually copied rather than re-encoded, the MP4 looks identical and appears in seconds.
For a film with surround sound, know that the converted AAC track is stereo. That is ideal for phones, laptops and TV speakers, but if you feed a 5.1 home cinema system, keep the MKV for that setup.
Usually not at all. When the MKV holds H.264 or HEVC video, which is the case for most OBS recordings and film releases, the frames are copied bit for bit into the MP4. Only rarer codecs such as VP9 or AV1, or HEVC with Maximum compatibility ticked, are re-encoded.
The picture is copied untouched, but the Dolby Digital (AC-3) soundtrack is turned into AAC because phones, browsers and many TVs can't play AC-3 inside an MP4. Converting a sound track is fast, so the job still takes only a little longer than a pure copy.
Only the main video and the main audio track, usually the first or default one. The other language, all subtitle tracks and the chapters are not carried over. If you need a specific language or burned-in subtitles, a desktop tool such as HandBrake lets you choose tracks, at the cost of a full re-encode.
Because MKV protects the recording. An MP4 interrupted by a crash or a power cut is normally lost, whereas an MKV remains playable up to the last second written. Converting afterwards takes seconds, so you get the safety of MKV and the compatibility of MP4.
Very likely, provided the video codec suits the TV. Many TVs reject MKV files or their DTS and AC-3 sound; an MP4 with AAC audio fixes both. If the film is HEVC and your TV is older, tick Maximum compatibility so the video becomes H.264.
When the video is copied, nothing is decoded, so even a full-length film is mostly limited by how fast your device can read and write the file and goes far faster than playback. Converting the audio adds a little time; re-encoding the video, when the plan says so, takes much longer.
No. Your browser reads the MKV from disk, rebuilds it as an MP4 inside the tab and saves it back to your device. Large films stay on your computer the whole time; the only practical limit is your device's memory.
Last updated October 28, 2026
iPhone videos are often HEVC. Copying is instant, but some Windows PCs, TVs and editors can't play HEVC.