Convert MP4 to MKV Online — No Upload, No Watermark
MKV is a flexible open container that holds multiple audio tracks, subtitle tracks, and chapter markers in one file — useful for archival or multilingual releases.
Drag & drop your video here, or click to browse
Max file size: ~2 GB (memory permitting)
Where MP4 files come from
MKV is the container people move to when they need it to hold more than MP4 allows, or when they want a recording that survives a crash. Media server libraries and OBS setups are the common destinations.
What changes when you convert MP4 to MKV
| Container | MP4 rewritten as Matroska |
|---|---|
| Video | Re-encoded to H.264 at CRF 23 |
| Audio | Re-encoded to AAC |
Worth knowing first
MKV can hold many audio tracks, chapters and styled subtitles, but converting from an MP4 does not create any of that. You get the same single video and audio pair in a more capable container. The capability only matters once you add tracks with a tool like MKVToolNix.
FAQ
Why would I want MKV over MP4?
Three reasons in practice: it can carry multiple audio and subtitle tracks, it supports styled ASS subtitles properly, and a partially written MKV still plays if the recording is interrupted. For a single video and audio track that never changes, MP4 is the more compatible choice.
Will my TV or phone play MKV?
Many modern smart TVs and Android phones do; iPhones and most browsers do not. If the file is destined for an Apple device or a web page, stay with MP4.
Is this lossless?
No. The video is re-encoded rather than copied across, which costs a little quality at CRF 23. A true remux would copy the stream untouched, and that needs desktop FFmpeg or MKVToolNix.
Is this MP4 to MKV converter really free and without upload?
Yes. The tool runs entirely in your browser using FFmpeg compiled to WebAssembly. Your MP4 file never leaves your device — open the DevTools Network tab to verify zero uploads. No signup and no watermark. The practical size limit is the memory your browser gives this tab, which is generous on a computer and a few hundred megabytes on a phone.