MKV to MP4
In your browser, no upload
MP4
rewrapped · not re-encoded
Video
MKV → MP4Specification
7 rows| Item | Value | Notes |
|---|---|---|
| Input | MKV, WebM | With H.264, HEVC or AV1 inside |
| Output | MP4 | The same tracks in an MP4 container |
| Length | Unchanged | Nothing is cut |
| Quality | Untouched | Nothing is decoded or encoded |
| Sound | Kept | AAC, MP3, AC-3, E-AC-3, Opus and FLAC copied; every track |
| File size | No limit | Read once, a few megabytes at a time |
| Processed in | This browser | Not sent to a server |
What happens
4What happens, and why
An MKV is a container. The picture and the sound inside it are usually H.264 or HEVC and AAC or AC-3 — exactly what an MP4 holds — and what an iPhone, a television or an editor refuses is the Matroska box around them, not the video. So this page reads where every frame is, writes a new MP4 index around the same bytes and copies them across. Nothing is decoded, which is why the badge says the file was not re-encoded and why a film is done in about the time it takes to read it from disk.
Two things are translated rather than copied, because the formats say them differently. Matroska stamps each frame with the moment it is shown; MP4 lists frames in decoding order with an offset, so those are worked out from the stamps. And the codec settings an MKV keeps in its track list go into the matching MP4 boxes — read from the first frame for AC-3 and E-AC-3, turned big-endian for Opus. Sound that starts a moment after the picture still starts a moment after it. The frames themselves are the source’s bytes.
What an MP4 cannot hold as it is stays behind, and the note under the badge names it: subtitle tracks, and sound in Vorbis, DTS, Dolby TrueHD or PCM. Several sound tracks — languages, a commentary — all come across, marked as alternatives so a player offers one at a time. A file whose picture is VP8, VP9 or MPEG-4 Part 2 (DivX, Xvid) cannot be copied into an MP4 that plays; it is refused with the codec named, and the WebM to MP4 page re-encodes it instead.
The file is never uploaded and never held in memory. It is read once, front to back, a few megabytes at a time, and the MP4 you download is assembled from the parts of it already on your disk, so there is no size limit beyond what this device can store.
FAQ
4Is this a real conversion
No, and that is the point. The video in an MKV is normally H.264 or HEVC, which an MP4 holds as it is. Re-encoding it would take many minutes and lose quality to no purpose. This writes a new container around the same frames instead.
Where did my subtitles go
They are left out, and the note counts them. MP4 keeps text subtitles in a different form, and turning one into the other is rewriting them rather than copying, so this page does not do it. Keep the MKV for its subtitles, or load the subtitle file next to the MP4 in your player.
Why was my MKV refused
Because its picture is in a codec an MP4 player does not play — VP8, VP9, or MPEG-4 Part 2 from DivX and Xvid — so a copy would be a file that does not open. The message names the codec. The WebM to MP4 page re-encodes those as H.264; it takes longer and says so before it starts.
Is the video uploaded
No. The browser reads the file from your disk and writes the new one on this page. There is no upload endpoint on this site, so there is nowhere for the file to go.