Free · no signup · never uploaded
MKV to MP4 Converter
Rewrap an MKV into an MP4 that plays anywhere — lossless and never uploaded.
Drop an MKV here or choose a file
or drop your file here.
MKV · converted in your browser, never uploaded
Runs in your browser — your file is never uploaded.
Short answer
To convert MKV to MP4, drop your file below and download the MP4. MKV usually already holds H.264 video, so the tool rewraps rather than re-encodes — fast, lossless, and entirely in your browser.
Why MKV files won’t play on your phone or TV
MKV (Matroska) is a flexible open container that can hold almost anything — multiple audio tracks, subtitle tracks, chapters. That flexibility is exactly why support is patchy: phones, smart TVs, browsers and upload forms often refuse .mkv outright, even though the video inside is ordinary H.264 they’d happily play. Moving that same stream into an MP4 wrapper is usually all it takes to make the file work everywhere.
What gets left behind
MP4 is stricter than MKV, so some extras don’t survive the rewrap. Multiple audio tracks are narrowed to the primary one, and MKV-style subtitle tracks aren’t carried over — MP4 handles subtitles differently. The video and main audio come through untouched. If your MKV holds something MP4 simply can’t store, the conversion reports an error rather than silently producing a broken file.
Video formats to MP4 — what happens to your file
| Input format | Usual codecs inside | Conversion | Max file size |
|---|---|---|---|
| MOV (QuickTime) | H.264 + AAC | Rewrap (lossless) | 500 MB |
| MKV (Matroska) | H.264 + AAC | Rewrap (lossless) | 500 MB |
| WebM | VP8/VP9 + Opus | Re-encode to H.264 | 100 MB |
| AVI | MPEG-4/DivX | Re-encode to H.264 | 100 MB |
| GIF | Animated GIF | Re-encode to H.264 | 200 MB |
A rewrap copies the existing video and audio streams into an MP4 container untouched — it is lossless and finishes in seconds. A re-encode decodes and re-compresses, so it takes longer and loses a little quality. Size caps differ because everything runs on one thread in your browser.
How this conversion works
Everything happens locally in your browser with ffmpeg — here is exactly how:
- Load the engine. ffmpeg compiled to WebAssembly loads once from this site — your file is never uploaded.
- Copy the streams. the existing video and audio are copied verbatim (-c copy) out of the Matroska container and into an MP4 one.
- Optimise for playback. the moov atom is written to the front (+faststart) so the MP4 plays and seeks immediately.
Assumptions
- Everything runs client-side; the streams are copied, never re-encoded.
- Quality is identical to the source.
- Subtitle tracks and secondary audio tracks are not carried over into the MP4.
Last reviewed: July 19, 2026
Frequently asked questions
How do I convert an MKV to MP4?+
Drop your .mkv in above and press convert. The existing streams are rewrapped into an MP4 container without re-encoding, so it’s fast and lossless — and runs entirely in your browser.
Are my videos uploaded to a server?+
No. Everything runs 100% in your browser using ffmpeg compiled to WebAssembly, so your file never leaves your device. Nothing is uploaded, stored, or sent anywhere — and once the converter has loaded it even works offline.
Does this lose any quality?+
No. The video and audio are copied into the new container untouched, so the picture and sound are identical to the source.
Will my subtitles come across?+
No. MKV subtitle tracks aren’t carried into the MP4, because the two formats handle subtitles quite differently. Extract them separately if you need them.
Why is the first conversion slower?+
The first time, your browser downloads the ffmpeg engine (about 31 MB) and starts it up. That happens once — later conversions reuse the loaded engine and are much faster.
What about multiple audio tracks?+
Only the primary audio track is kept. MKV commonly carries several languages; MP4 output here takes the first one.
Related tools
Video is processed entirely in your browser with ffmpeg and never uploaded.