How to Convert MP4 to MKV Online Free

Both MP4 and MKV are containers, not codecs. A container is the box the video and audio streams sit in; the codec is how those streams are actually compressed. MKV, short for Matroska, can hold everything an MP4 holds, including H.264 and H.265 video with AAC audio, and quite a bit more besides.^[3]

Because the two boxes accept the same contents, converting MP4 to MKV usually means copying the existing streams straight across into a new container. Nothing gets decoded and re-encoded, so the picture and sound come out bit for bit identical to the original. This guide explains why that lossless rewrap is possible, exactly what happens during it, and when MKV's extra flexibility is worth trading away MP4's near-universal reach.

Why this is a rewrap, not a re-encode

The technical term for this is remuxing, or a stream copy. The converter reads the video and audio packets already inside your MP4 and writes them, untouched, into an MKV wrapper. Since the H.264 or H.265 frames are never re-compressed, there is no generation loss and no quality decision to make. It is also fast, because the work is closer to a file copy than to a full encode: the tool is shuffling already-compressed data, not doing the heavy arithmetic of encoding.

You only need a true re-encode if you also want to change the codec itself, for example turning H.264 into H.265 to save space. That is a separate operation with its own quality cost. Straight MP4 to MKV does not require it, and you should avoid re-encoding unless you have a specific reason, since every re-encode throws away a little detail that you can never get back.

Container versus codec, and how each is built

The reason this works comes down to a clean separation that both formats respect: the container and the codec are independent layers. A codec such as H.264 defines how a sequence of pictures is compressed into a stream of bytes. A container defines how one or more such streams, plus audio, subtitles, and timing metadata, are packaged into a single file. Because the compressed video stream is the same object regardless of which box it sits in, moving it between compatible containers changes nothing about the video itself.

MP4 is built on the ISO base media file format, with a box-based structure and a single index that maps every frame.^[2] Matroska is built on a flexible binary structure called EBML, a kind of binary XML, whose element-based design was created deliberately to be open-ended and extensible.^[4] That extensibility is why MKV can carry an almost unlimited variety of streams and metadata, where MP4's more rigid design accepts a narrower set. The Library of Congress catalogs MP4 and Matroska side by side as modern multimedia containers, differing mainly in flexibility and in breadth of device support.^[1]

How the remux works under the hood

A remux is a short, mechanical pipeline. The converter first demultiplexes the MP4, reading its index and pulling out each compressed stream, the video, every audio track, and any timed metadata, as a sequence of intact packets. Nothing is decoded; the packets stay exactly as they were encoded. It then multiplexes those same packets into a Matroska structure, wrapping each in the EBML elements MKV uses and writing a new set of timestamps and seek information so playback and scrubbing work correctly in the new file.

The only genuinely new content the converter writes is organizational: the MKV's own index, track descriptions, and any extra streams you choose to attach at this stage. Because the expensive part of video processing, encoding pixels, is skipped entirely, the whole operation runs at close to disk speed and the video and audio are preserved to the byte. This is also why you can rewrap back and forth between MP4 and MKV as many times as you like without ever degrading the picture.

One consequence of remuxing rather than re-encoding is that the file size barely changes. Since the compressed video and audio are the same bytes in either container, an MKV of a given MP4 is almost exactly the same size, give or take the small overhead of the container's own structures and any extra tracks you add. If you ever see a large size difference after a supposed rewrap, it is a sign the tool actually re-encoded rather than copied, which is worth checking because an unintended re-encode both wastes time and quietly costs quality. A true MP4-to-MKV remux should finish quickly and land within a whisker of the original file size.

What MKV gives you that MP4 cannot

If the quality is identical, why bother? Because MKV is a far more flexible container. It was designed to bundle many streams into one file, which makes it the go-to format for archiving and for multi-language releases.

So you convert to MKV when you want to keep several audio languages together, attach external subtitle files as switchable tracks rather than burning them into the picture, or store chapter markers. For a long-term archive of a film with commentary and subtitles, MKV keeps everything in a single tidy file. There is one more reason it dominates archival use: MKV is an open, well-documented, community-maintained format whose specification is published for anyone to implement, which makes it a safe bet for files you intend to keep for decades. Its extensible design also means it can adopt new codecs as they appear without the container itself needing to change, so a Matroska file from years ago still opens with today's tools and can hold tomorrow's video.

MP4 is not weak here, but it was shaped by different priorities. Because MP4 is the format browsers, phones, and streaming services standardized on, its design favors broad, predictable compatibility over open-ended flexibility. That is a genuine strength for delivery and a limitation for archiving, and it is exactly why the two formats coexist: MP4 to reach everyone, MKV to keep everything.

Convert MP4 to MKV, step by step

1. Open the converter and add your MP4

   Open the FileFormer video converter and drop in your MP4. It runs on your device, so the file is not uploaded anywhere.

2. Select MKV as the output

   Choose MKV. Since it accepts your file's existing video and audio, the tool copies the streams across instead of re-encoding them.

3. Attach any extra tracks

   This is the moment MKV earns its keep. If you have separate subtitle files or an extra audio language, add them so they live inside the one MKV.

4. Remux and download

   Run it and save the result. The rewrap finishes quickly because no frames are re-encoded, and the output matches your source exactly.

Working with extra tracks and subtitles

The reason to choose MKV over a straight MP4 is almost always its handling of extra tracks, so it is worth knowing how they behave. MKV treats every stream as a first-class, labeled track: you can bundle several audio languages, mark one as the default, and flag another as commentary, and a player will offer them all in a menu. This is how a single MKV of a film can carry the original audio, a dubbed language, and a director's commentary without any of them being welded onto the others.

Subtitles are the standout case. MKV supports soft subtitles, meaning the subtitle text is stored as its own selectable track that the viewer can turn on or off, rather than being burned in permanently onto the picture. Soft subtitles keep the video clean, let the viewer pick a language, and can be styled or repositioned, and MKV can hold many of them at once. When you attach an external subtitle file during the rewrap, it becomes one of these switchable tracks inside the single MKV, which is exactly the tidiness that makes MKV the archival favorite.

Real-world scenarios and the right call

Whether MKV or MP4 is the better target depends on what you are trying to do. A few common cases:

The dividing line is control. When you control the playback environment, or when preserving multiple tracks matters, MKV wins. When the file has to play for anyone, anywhere, on the first try, MP4 wins. Because the rewrap is lossless in both directions, you are never locked in.

A practical workflow that many people settle on is to keep two files that share the same underlying video. The MKV is the master: it holds every audio language, every subtitle track, and the chapter markers, and it lives in your archive untouched. From that master you generate a plain MP4, carrying just the main audio and burned or embedded subtitles as needed, whenever you have to send something to a phone, a browser, or a colleague. Since both are produced by lossless remuxing from the same streams, neither ever degrades, and you get the best of both formats at once: the completeness of Matroska for keeping and the reach of MP4 for sharing. Set up that way, the question of "MKV or MP4" stops being a choice and becomes simply which copy you reach for.

The one downside to weigh

MKV's flexibility comes at the cost of reach. MP4 plays natively on almost every phone, browser, smart TV, and editing app; MKV does not. Some phones, some browsers in-page, and a number of older televisions will refuse an MKV or play only part of it. If your goal is simply to hand a clip to someone and have it play with zero friction, keep the MP4.

Reach for MKV when you control the playback environment, or when the extra tracks and archival tidiness matter more than universal support. If you later need broad compatibility again, you can rewrap the same MKV back to MP4 just as losslessly, dropping only whatever tracks MP4 cannot carry. This two-way, lossless relationship is what makes the pair so practical: keep your master as an MKV with every track intact, and generate a slim MP4 whenever you need something to play everywhere. Neither direction ever touches the video itself, so you can move between them as often as your situation demands without paying a quality price.