Convert MKV to MP4
Convert Matroska MKV files to MP4 for TVs, phones and editors. The streams are usually copied straight across, so even a 4 GB film takes seconds.
- Runs in your browser
- Files are never uploaded
- Free, no account
First use downloads the 10 MB video engine from this site. After that it is cached and works offline. It is WebAssembly served from onthisdevice.com — no third-party CDN — and your file is still processed entirely on your own device.
Turning a Matroska file into something your TV will accept
MKV is the most capable video container in common use: it can hold any codec, any number of audio tracks, chapters, attachments and several kinds of subtitle at once. That flexibility is exactly why hardware refuses it. Smart TVs, games consoles, older Android phones and most upload forms only implement MP4, and they reject the file on the wrapper alone — usually while the video inside is ordinary H.264 that the device could decode without breaking a sweat.
Swapping the wrapper is therefore the whole job, and it costs nothing in quality. FFmpeg reads the Matroska index, copies every compressed packet across and writes an MP4 index in front of it. No frame is decoded.
What survives the conversion, and what does not
| Inside the MKV | In the MP4 | Why |
|---|---|---|
| H.264 or HEVC video | Copied unchanged | MP4 carries both natively |
| AAC or MP3 audio | Copied unchanged | Both are standard MP4 audio |
| FLAC, DTS or Vorbis audio | Re-encoded to AAC 160 kbps | MP4 has no slot for them |
| VP9 or AV1 video | Re-encoded to H.264 (slow) | Players expect these in WebM |
| Second and third audio tracks | Dropped — the first track is kept | Track selection is deliberately out of scope here |
| Soft subtitles (SRT, ASS, PGS) | Dropped | MP4 has no equivalent for ASS or PGS |
| Chapters and attachments | Dropped | Not carried by the copy path |
If subtitles matter to you, the honest answer is to keep the MKV and play it in VLC or MPV rather than convert it — burning subtitles into the picture is a re-encode and it makes them permanent, which is rarely what people want.
Very large files
This is the one place where a browser tool has a real constraint worth stating plainly. The input and the output both live in the tab’s memory, and browsers give WebAssembly roughly 2 GB to work with. On the copy path there is no decoding buffer on top of that, so multi-gigabyte films usually go through on a desktop with free RAM; on a phone, or with thirty other tabs open, they may not. If memory runs out the page tells you that in a sentence, rather than hanging on a spinner.
Compare that with the alternative: uploading 4 GB to a converter that caps free accounts at 1 GB, waiting through a queue, then downloading 4 GB back. The transfer is the expensive part of that workflow, and it is the part that disappears here.
Does converting MKV to MP4 lose quality?
Not when the streams are copied — the output is bit-for-bit the same video data in a different envelope, and the file size lands within a fraction of a percent of the original. The page tells you before it starts whether your file qualifies. Quality is only ever lost when something inside the MKV has no MP4 equivalent, and in that case you get a clear warning and a choice, not a silent downgrade.
Related jobs
If you only wanted the soundtrack, MKV to MP3 skips the video entirely. If the file is too big for a messaging app, compress video targets a size. If it is too long, trim video cuts it without re-encoding.
Questions
Why will my TV not play MKV in the first place?
MKV is a container, not a codec, and it is the one format most smart TVs, PlayStations and older phones never licensed. The video inside is very often plain H.264 that the device can decode perfectly well — it simply refuses the wrapper. Swapping the wrapper is all that is needed.
What happens to subtitles and extra audio tracks?
The default video and audio streams are carried over. Soft subtitles and secondary language tracks are not copied, because MP4 has no equivalent for MKV subtitle formats such as ASS or PGS. If you need those, keep the MKV and use a player that supports it, such as VLC.
Can it handle a 4 GB or 8 GB film?
On a desktop with plenty of free memory, usually yes, and on the copy path it is quick because nothing is decoded. Very large files are the one case where a browser tool can hit a wall: the whole input and output live in the tab’s memory. If it fails, the page says so plainly rather than hanging.
Does converting MKV to MP4 reduce quality?
Not on the copy path — the video data is bit-for-bit the same, so the result is indistinguishable from the source. Quality is only lost when the MKV contains something MP4 cannot hold, such as VP9 video or FLAC audio, and the page warns you before that happens.