Why MKV to MP4 is usually a copy, not a conversion
MKV (Matroska) and MP4 are both containers: boxes that hold a video stream, one or more sound streams and, in MKV’s case, often subtitles and chapters too. The video inside an MKV is nearly always H.264 or HEVC, sometimes VP9 or AV1, and an MP4 can hold all four. So the picture does not need to be decoded and compressed again. It is lifted out of one box and put into the other, frame for frame. That is why this takes seconds instead of minutes, and why the quality cannot change.
After copying, the tool proves it: it reads every video frame of the new MP4 and compares it with the original, byte for byte, and tells you how many matched.
What we measured
We converted test files in Chrome on a Windows PC on 16 October 2026 and checked every MP4 with a separate reader (PyAV), not with this tool:
| MKV | Time | Picture | Sound in the MP4 |
|---|---|---|---|
| 60 s, 1080p, H.264 + AAC, 60.8 MB | 0.22 s | 1,800 of 1,800 frames identical | AAC, copied |
| OBS-style recording, H.264 + AAC | 0.05 s | 120 of 120 identical | AAC, copied |
| Film: H.264, Dolby Digital 5.1 (Turkish) + AAC (English), 2 subtitle tracks | 0.34 s | 120 of 120 identical | both tracks, languages kept; Dolby converted to AAC |
| HEVC + Dolby Digital Plus (E-AC-3) | under 0.5 s | identical | converted to AAC |
| VP9 + Opus (a downloaded web video) | under 0.1 s | identical | Opus, copied |
| H.264 + FLAC | under 0.1 s | identical | FLAC, copied |
The size of the MP4 is almost exactly the size of the MKV (60.83 MB from 60.83 MB), because nothing in the picture changed. Converting to MP4 does not make a video smaller; for that, use Compress Video afterwards.
The MP4 has no sound: Dolby Digital
Films, TV recordings and Blu-ray copies often carry their sound as Dolby Digital (AC-3) or Dolby Digital Plus (E-AC-3). MP4 can hold both, and a simple copy keeps them. The trouble is playback. We played MP4s with Dolby sound in Chrome on Windows: the picture ran, no error appeared, and the browser decoded exactly 0 bytes of sound. Many phones do the same. Edge on the same PC played the sound, because it borrows Windows’ own Dolby decoder. That is the most common complaint about MKV to MP4: the picture plays and there is no sound, in some players and not others.
So by default this tool decodes Dolby sound and converts it to AAC, which plays everywhere, while the picture is still copied untouched. If the MP4 is only for a TV or a home cinema receiver that handles Dolby, choose Keep as Dolby. A 5.1 track becomes stereo in Chrome, which cannot write 5.1 AAC. We tested that the dialogue survives: in a 5.1 test file with sound only in the centre channel, where films put the voices, the stereo result carried it at the standard −3 dB on each side (−12.0 dBFS from −9.0).
Subtitles: why converters lose them, and what this one does
An MKV can carry several subtitle tracks inside the file. When we converted our test film with Mediabunny, the open-source video library this page is built on, the subtitle tracks did not even appear in its list of tracks: they were dropped without a warning. MP4’s own text subtitle format cannot be written by that library either, and players differ in whether they show it anyway.
This tool reads the subtitles from the MKV itself and saves each text track as a separate .srt file,
named like the video with the language added (film.eng.srt). Put it in the same folder as the MP4 and
VLC, MPC, Plex, Kodi and most smart TVs load it automatically. Styled ASS subtitles, common in anime, are saved
twice: as plain .srt and as the original .ass with its fonts, colours and positions. We checked the timings to the
millisecond against the original, including two lines on screen at once and text containing commas.
Subtitles stored as pictures cannot become text this way: PGS from Blu-ray and VobSub from DVD are images of letters, and turning them into text needs character recognition. The tool lists them and says so instead of pretending.
Several languages in one file
The tool lists every sound track with its language and format. Tick the ones to keep. All kept tracks go into the MP4 with their language labels, and players such as VLC let you switch between them. Most players start with the first track, so if you only ever want the English sound, untick the others.
OBS recordings
OBS records to MKV by default for a good reason: if the computer or OBS crashes in the middle of a recording, an MKV is still playable up to that moment, while an MP4 that was being written usually is not. Video editors and upload forms prefer MP4, so the usual habit is to record MKV and convert afterwards. If OBS is open, its own File → Remux Recordings does the same copy as this page. This page is for when OBS is not to hand, or when you have received someone else’s recording.
Large files
Up to 1.5 GB the MP4 is built in the browser’s memory and offered as a download. Above that, Chrome and Edge ask where to save the MP4 and write it straight to that file, so a long film does not need its size in free memory. Other browsers build big files in memory, which can fail on phones and older computers. Either way the file is never uploaded: everything happens on your device.
When to choose “Most compatible”
- HEVC (H.265) video plays on recent phones, Macs and TVs, but not on every Windows PC or upload form.
- VP9 and AV1, common in videos downloaded from the web, play in Chrome, Edge and VLC, but Apple devices, QuickTime and many TVs may not play them.
For those, Most compatible decodes the picture and encodes it again as H.264, which plays almost everywhere. It takes much longer than copying and changes the picture very slightly, so use it only when the copied MP4 does not play where you need it. For iPhone videos, use MOV to MP4; for browser screen recordings, WebM to MP4; to cut the start or end without re-encoding, Trim Video.