MP4 to MKV
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Drop your .mp4 file here
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MP4 to MKV moves your video's existing H.264 (or HEVC) and AAC streams into a Matroska container instead, without decoding or recompressing anything. It's a pure remux — FFmpeg copies the compressed packets directly from one wrapper to another — which is why it's one of the fastest conversions this toolkit offers regardless of how long or high-resolution the source file is.
The reason to bother at all comes down to what MP4 as a container format was and wasn't designed to do. MP4 (technically ISOBMFF, the ISO Base Media File Format) was built with a fairly rigid, standards-committee structure aimed at broad, predictable compatibility — one primary video track, one primary audio track, basic subtitle support that many players implement inconsistently, and no real concept of chapter markers as a first-class feature. Matroska (MKV) was designed later with the opposite priority: maximum flexibility. A single MKV file can legitimately hold multiple audio tracks — an English mix and a director's commentary, say, or dubbed audio in several languages — multiple subtitle tracks a player can switch between, embedded chapter markers with named chapter points, and even attached fonts or cover art, all inside one container that any MKV-aware player understands as a matter of course.
This matters most concretely for home media server setups: Plex, Jellyfin, Emby, and Kodi-based home theater PCs are all built around the assumption that a well-organized library uses MKV for anything with multiple audio or subtitle tracks, because the format handles track selection and metadata far more gracefully than MP4 ever was designed to. If you've recorded a video with a separate commentary track, need to attach subtitles in more than one language, or want your home server to display proper chapter navigation, remuxing to MKV first gives you a container that actually supports adding those things cleanly — the remux itself doesn't add tracks, but it puts you in a format where subsequent tools (like MKVToolNix) can.
There's also a licensing angle worth knowing: Matroska is a fully open, patent-unencumbered container specification, maintained by a nonprofit foundation, with no royalties or licensing restrictions tied to using the container itself (note this is separate from the video codec inside it, which may have its own licensing considerations). That openness is part of why so much open-source media software — VLC, Kodi, Jellyfin, HandBrake — treats MKV as a first-class citizen. Everything here happens locally through FFmpeg WebAssembly, so your video never leaves your browser during the remux.
FFmpeg reads the compressed video and audio packets directly from the MP4's structure and writes them into a new Matroska (MKV) container using stream copy mode (-c copy), without decoding or re-encoding either stream. Because it's a pure container swap, the operation is limited by file read/write speed rather than encoding computation, so even a large 4K file typically remuxes in a few seconds.
Upload your MP4 file
Drop an .mp4 file onto the upload area — any standard H.264 or HEVC encoded video works.
Confirm the file
Check the filename and size to make sure it's the video you intend to remux.
Click Convert to MKV
FFmpeg copies the video and audio streams directly into a new Matroska container without re-encoding.
Wait a few seconds
Because this is a stream copy, even long or high-resolution files finish converting almost immediately.
Download the MKV file
Save the output — it keeps your original filename with a .mkv extension.
Import into your media server if desired
Drop the MKV into your Plex, Jellyfin, or Kodi library, or use MKVToolNix afterward to add extra audio or subtitle tracks.
Convert more files as needed
Repeat for as many MP4 files as you want to add to your MKV-based library.
Multi-Language Subtitle Libraries
Remux to MKV as the foundation for adding multiple subtitle language tracks that a viewer can switch between in one file.
Multiple Audio Tracks
MKV comfortably holds several audio streams — original plus commentary, or dubbed audio in different languages — inside a single file.
Plex & Jellyfin Home Servers
Home media server libraries are generally built around MKV's richer metadata and multi-track handling rather than MP4's simpler structure.
Kodi & HTPC Setups
Home theater PC software like Kodi takes full advantage of MKV's native chapter markers and embedded metadata that MP4 handles poorly.
Open-Format Archiving
Choose MKV for long-term storage when you want a container specification that's open, patent-free, and not tied to any one company.
Chapter-Marked Long-Form Video
MKV supports named chapter points natively, useful for long recordings like lectures or livestream VODs you want viewers to navigate easily.
Zero-Loss Stream Copy
Video and audio packets move into the new container completely unchanged — no recompression, no quality difference.
Open, Patent-Free Container
Matroska carries no licensing fees or patent encumbrances on the container format itself, unlike some proprietary alternatives.
Built for Multi-Track Media
MKV natively supports multiple audio tracks, multiple subtitle tracks, chapters, and attachments in ways MP4 was never designed for.
Runs Entirely in Your Browser
FFmpeg WebAssembly performs the remux locally — your video file is never sent to a server.
Fast Regardless of File Size
Since nothing is re-encoded, even large 4K files typically remux in a matter of seconds.
Free, No Watermark
Free with no account for files under 40 MB; sign up free for 80 MB or go Pro for 250 MB — output is never watermarked.
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