Convert AVI to MKV
Move an old AVI into an MKV container without re-encoding anything. The video and audio streams are copied straight across, so the result is bit-for-bit the same picture and sound and it finishes in seconds. This does not modernize the codecs inside — it puts them in a container that can carry subtitles, several audio tracks and proper timing, which AVI cannot.
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Up to 50 MB. Deleted automatically after 30 minutes.
What it does
- Streams copied, not re-encoded — identical quality
- Fixes AVI's audio-drift problem
- MKV carries subtitles and extra audio tracks
- No watermark and no sign-up
How to use AVI to MKV
- 1
Add the AVI
Drop the file in or choose it from your computer. Old AVI files are usually small enough to upload whole.
- 2
Convert
The video and audio streams are copied into an MKV with `-c copy`. Nothing is decoded and nothing is encoded.
- 3
Use it
The MKV plays in VLC, MPV and every media server, and will now accept subtitle and audio tracks.
The container changes; the video does not
An AVI file is two things: the encoded picture and sound, and a 1992 Microsoft container holding them. This conversion replaces the second and leaves the first exactly as it was — a copy, not a conversion, verified by comparing the video stream's checksum before and after.
That means it does not help with the problem most people have with old AVI files. DivX and Xvid stay DivX and Xvid, and a phone or a browser will still refuse to play them. For that, convert to MP4, which re-encodes to H.264.
What it does help with is everything the container was never able to express.
How it works
The video and audio streams are copied into an MKV container with -c copy. No
frame is decoded and no encoder runs, so the operation is bounded by reading and
writing the file rather than by how long the video is.
That the copy is genuinely lossless was checked rather than claimed: the video stream's checksum is identical before and after.
What MKV can carry that AVI cannot
Subtitles are the clearest case. AVI has no real subtitle support, so captions added to one are almost always burned into the picture — permanent, untranslatable and unable to be turned off. MKV stores them as selectable text tracks, several at once.
The same applies to audio: a second language, a commentary, an original mix alongside a dub. And to chapter markers, which AVI simply has no concept of.
The sync problem, and why a remux fixes it
AVI predates variable-bitrate audio as a normal thing and has no proper way to describe one. Its timing model assumes audio arrives at a constant rate. An enormous amount of variable-bitrate MP3 was put into AVI files regardless.
The result is the familiar symptom: sound and picture in step at the start, and visibly apart by the end, the error accumulating as the file runs.
MKV timestamps each block explicitly rather than assuming a rate, so the remux writes correct timing for audio that always had it — the information was there, and the old container had nowhere to put it. If you have a recording where the voices lag behind the mouths, this is very often the entire fix, and it costs no quality at all because nothing is re-encoded.
When to convert instead
If the goal is a file that plays on a phone, in a browser, or on a smart TV, this is the wrong conversion — those need H.264, and a remux does not provide it. Use AVI to MP4 for that. Use this when the codecs are fine for wherever it is going and the container is what is in the way.
Examples
An old recording going into a media server
Plex, Jellyfin and Kodi index MKV properly and expose its tracks. AVI works less consistently and carries no useful metadata.
A file that needs subtitles adding
AVI has no real subtitle support, so captions usually end up burned into the picture. MKV holds them as text a viewer can switch off.
Frequently asked questions
Does this improve the video?
No, and it is important to be clear about that. The picture and sound are copied unchanged, so a DivX or Xvid file stays DivX or Xvid — with all the compression artifacts it has always had. What changes is the container around them. If you want the file to play on a phone or in a browser, convert to MP4 instead, which re-encodes to H.264.
Then why do it?
For what MKV can hold. Subtitles as selectable text, several audio tracks, chapter markers and proper timing — none of which AVI supports meaningfully. It is the right first step before adding captions to an old recording, and it makes the file behave properly in a media server.
Will it fix audio that drifts out of sync?
Usually, yes, and this is the underrated reason to do it. AVI has no way to describe variable-bitrate audio and a great deal of variable-bitrate MP3 was put into AVI files anyway — which is why so many drift, imperceptibly at the start and badly by the end. MKV timestamps each block properly, so the remux rewrites the timing correctly.
Is the file the same size?
Almost exactly. The streams are identical and only the container differs, so any change is the few kilobytes of structural overhead. A large difference would mean something was re-encoded, which is not what this does.