Both AVI and MKV are containers: wrappers that hold a video stream, one or more audio streams, and metadata. They are not codecs, so converting from one to the other does not automatically re-encode the picture. In most cases the video and audio streams are simply lifted out of the AVI wrapper and dropped into an MKV wrapper untouched. That operation is called a remux, and it is nearly instant and completely lossless.
The reason you would bother is that AVI is a container from the 1990s with real limits, while MKV is a flexible modern format that can hold almost anything you throw at it. This guide explains how each container is built, exactly what happens during a remux, when a slower re-encode becomes unavoidable, and how to decide between MKV and its main rival, MP4.
Container swap, not a re-encode
When the conversion is a remux, not a single pixel changes. The exact same encoded frames that were inside the AVI end up inside the MKV, bit for bit. This is why AVI to MKV is usually one of the quickest conversions you can do: the tool is copying streams, not decoding and re-drawing every frame. A long film can finish in seconds rather than minutes.
This only works cleanly when the streams inside the AVI are ones MKV accepts, which is nearly everything, since MKV was designed to be a universal container.[3] The classic case is an AVI carrying an older codec like DivX or Xvid, which MKV holds without complaint.
AVI to MKV is normally a lossless repackage. The picture stays identical and the conversion is fast, because nothing is re-encoded.
AVI and MKV: two containers, decades apart
AVI, Audio Video Interleaved, was introduced by Microsoft in 1992 as part of its Video for Windows platform. It is built on the Resource Interchange File Format, the same tagged-chunk structure that underlies WAV, and it interleaves blocks of video and audio through the file so a player can read them in step.[1] That design was excellent for its era, but it predates most of what we now expect from video. AVI's indexing and timing model assumes a constant frame rate, has no clean way to embed modern subtitle formats, and handles multiple audio tracks awkwardly.[2]
MKV, the Matroska container, was created around 2002 with the explicit goal of being a universal, open, future-proof wrapper.[3] Instead of RIFF chunks it uses a flexible binary structure called EBML, a binary cousin of XML, which lets it describe an essentially unlimited variety of tracks, chapters, tags, and attachments.[4] The practical upshot is that almost any stream AVI can hold, MKV can hold too, plus a great deal AVI never could. Moving from one to the other is therefore usually a straight copy into a much more capable box.
How the remux works under the hood
A remux is a demux followed by a mux, with no codec in between. First the tool demultiplexes the AVI: it reads the RIFF structure, identifies each stream, and separates the interleaved video and audio back into distinct encoded elementary streams. Crucially it does not decode those streams into raw frames; it keeps them exactly as encoded.
Then it multiplexes them into a Matroska file, writing the compressed video and audio into EBML blocks and adding a header that declares the codecs, timing, and any extra tracks. Because the encoded frames are copied verbatim, the video and audio come out numerically identical to the source. The only thing that has genuinely changed is the packaging and the metadata around the streams. This is why a remux both finishes almost instantly and preserves quality perfectly: the expensive, lossy work of decoding and re-encoding never happens.
Why move AVI into MKV
The upgrade is about what the container can carry. AVI struggles with modern subtitles and handles multiple audio tracks awkwardly, and its timing model is dated enough that variable frame rate video can drift out of sync. MKV was built to solve exactly these problems.
| Capability | AVI | MKV |
|---|---|---|
| Subtitle tracks | Poor, external only in practice | Many, embedded |
| Multiple audio tracks | Clumsy | Built in |
| Chapters and metadata | Minimal | Rich |
| Modern codecs | Limited | Almost anything |
If you want one file that bundles the movie, a second language track, and a subtitle file together, MKV is the natural home and AVI simply cannot do it neatly.
There is also a preservation argument. AVI is a legacy format that fewer modern tools support cleanly, and its indexing can become fragile in files that were edited or partially downloaded over the years. Matroska is openly documented, actively maintained, and designed to degrade gracefully, so moving an old AVI's streams into MKV is a way of rehousing valuable footage in a container that is far more likely to remain readable for the long term.[4] Because the remux is lossless, you gain that durability without paying any quality cost.
If the conversion finishes in seconds, it was a remux and your picture is untouched. If it takes minutes and your device works hard, the codec was changed and a re-encode happened, which costs a little quality.
Convert AVI to MKV, step by step
Open the converter and add your AVI
Open the FileFormer video converter and drop your AVI in. It all runs on your device, so the file is never uploaded.
Select MKV as the output
Choose MKV. If the AVI holds a stream MKV supports, the streams are copied straight across with no quality loss.
Attach extra tracks if you have them
This is the moment MKV earns its keep. Add an extra audio language or a subtitle file and they ride along inside the single output file.
Convert and download
Export the MKV. A remux finishes quickly; download and the file is ready to play.
When a re-encode is unavoidable
There is one situation where AVI to MKV takes longer and costs a little quality: if you also ask to change the codec, for example to shrink a bulky Xvid file into modern H.264. That is no longer a remux. Every frame has to be decoded and encoded again, which takes real time and loses a small amount of detail, as all lossy re-encoding does. If your only goal is a better container, skip re-encoding and keep the straight copy. If you specifically want a smaller file, re-encoding is the price, and you may prefer targeting MP4 for wider device support instead.
The reason MKV so rarely forces a re-encode is worth stating plainly: it accepts almost every codec in existence, so whatever is inside the AVI usually has a home in Matroska as-is. Contrast that with converting the same AVI to MP4, where an old DivX or Xvid stream that MP4 will not store must be rebuilt in H.264. If you value speed and perfect fidelity above universal device support, MKV is the container that lets you copy rather than re-encode.
What MKV can carry that AVI cannot
The reason MKV is worth moving to becomes concrete once you look at its track model. Matroska treats a file as an ordered set of tracks of arbitrary types, each with its own language tag, name, and flags marking it as default or forced.[4] That single idea unlocks a great deal that AVI handles poorly or not at all: several full audio tracks so a film can hold both its original language and a dub, multiple subtitle tracks in different languages that a viewer can switch between, and even attached fonts or cover images that travel inside the same file.
On top of tracks, MKV supports chapters, letting a long recording carry named jump points, and rich tags for titles, dates, and other metadata. It also handles variable frame rate video correctly, which matters for screen recordings and phone footage where the frame rate is not constant and where AVI's fixed-rate timing model can cause audio to drift out of sync over a long clip. None of this changes the picture during a remux; it simply means the container around your untouched video and audio can now describe and bundle far more than AVI ever could.
Real-world scenarios and the right choice
Whether a remux is enough, or a re-encode is worth it, depends on what you are trying to achieve:
| Goal | Recommended approach |
|---|---|
| Archive an old DivX or Xvid AVI safely | Remux to MKV. The picture is preserved bit for bit and lives in a modern, well-documented container. |
| Bundle a film with a second language and subtitles | Remux to MKV and attach the extra tracks. AVI cannot do this neatly; MKV was built for it. |
| Play the file on a phone, tablet, or smart TV | Consider MP4 instead. MKV support on consumer hardware is patchier than MP4. |
| Shrink a large old AVI to save space | Re-encode to a modern codec. This is lossy and slow but is the only way to reduce size. |
| Feed the file into an editor | Check which container the editor prefers; many read MKV, but some pipelines still expect MP4. |
MKV versus MP4 and keeping the AVI
MKV is the most capable container available, but it is not always the right target. Its main rival, MP4, is less flexible but far more widely supported on phones, tablets, browsers, and televisions. If your priority is that the file simply plays wherever you send it, MP4 usually wins, even though it accepts fewer codecs and handles subtitles and multiple tracks less gracefully than MKV. The AVI to MP4 guide covers that path.
It is also worth asking whether to convert at all. If the software you use already opens AVI and you have no need for embedded subtitles or extra tracks, keeping the original AVI is perfectly fine and costs nothing. Convert to MKV when you want a flexible, future-proof home for the streams with no quality loss, choose MP4 when broad playback compatibility matters most, and leave the AVI alone when it already does everything you need.
Repackage your AVI as MKV now
Copy the streams into a flexible MKV container in your browser, with nothing uploaded.
Key takeaways
- AVI and MKV are containers, so this is usually a lossless remux, not a re-encode.
- The picture stays identical and the conversion is fast when codecs are compatible.
- MKV adds embedded subtitles, multiple audio tracks, and chapters that AVI cannot handle well.
- Only changing the codec triggers a slower, slightly lossy re-encode.