Convert an MP4 video to MKV (Matroska) losslessly and locally - for multiple audio tracks, subtitles and archiving. No upload.
Running locally on your device ...
Running locally on your device ...
0%
Your files never left your device
No. Everything runs in your browser - your file never leaves your device. How this is verifiable
MP4 plays everywhere, but as a container it is comparatively narrow: it allows multiple audio tracks, but handles subtitles and chapter markers more awkwardly than MKV (Matroska), an open container built for exactly these cases - popular for archiving, multi-language videos and fansub projects.
Because MKV's lossless copy set is a superset of MP4's own (the same H.264/H.265/AAC family plus considerably more), this conversion is always a pure, lossless repackage - regardless of what the MP4 contains in detail. Every track (picture, every audio track, every subtitle track) is copied 1:1, nothing is recomputed.
In practice that means: if your MP4 has two audio tracks (e.g. an original and a commentary track), both survive in the MKV - MP4 itself already makes that harder with some players. Everything runs locally in the browser, with your video never uploaded.
| Input formats | MP4, M4V |
|---|---|
| Output format | MKV |
| Batch processing | No |
| Processing | Locally in your browser (WebCodecs) |
| File upload | None |
Limitations: Always a lossless repackage with no quality loss, including every audio and subtitle track - no re-encode needed, finishes in seconds. One file per run, ~1 GB limit (4 GB/tab in the browser).
No, the entire conversion happens locally in your browser.
No, MP4 to MKV with this tool is always a lossless repackage, not a re-encode.
For example to handle multiple audio tracks, subtitles or chapter markers more reliably, or for archiving and fansub purposes.
Yes, every track of the MP4 is copied 1:1 into the MKV.
Yes, use the sibling tool "Convert MKV to MP4".