What is an AVI file?

AVI is a container: a wrapper that holds streams encoded by something else. It is used for archiving.

The extension is .avi, and the full name is Audio Video Interleave. Both matter less than what the file can hold, which is what the rest of this page is about.

Microsoft published it in 1992.

A format that has been readable for that long is a format worth trusting with something you want back in ten years.

It is written down and implementable, though the vendor decides what the next version looks like. In practice that is enough for the file to keep opening; it is not enough to be sure of it.

It still opens everywhere and is still written by older tools, but nothing new is being built around it. Convert what you mean to keep, and do not choose it for something starting today.

It usually holds MJPEG, DivX and Xvid. This is the distinction behind most "why will this not play" questions: two files with the same extension can contain entirely different encodings, and a player that refuses one may be perfectly happy with the other.

It also explains why changing container is often fast and lossless while changing codec is neither. Moving the same stream into a different box rewrites the box; re-encoding rewrites the picture.

AVI can hold several audio tracks.

It has no place for subtitles, which is the thing most often lost on the way out of a richer container.

VLC and HandBrake read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

AVI is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.

The recurring complaints: the files are large for what they contain and support outside its home ground is uneven.

None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.

Microsoft introduced AVI with Video for Windows in 1992, before web video, before streaming, before most of what a video file is now expected to do. It is a container — a box holding a video stream and an audio stream side by side — and the box itself is nearly as old as the personal computer era.

Almost every difficulty with AVI traces back to that date. The design predates the problems it is now asked to solve, and rather than being bad at them it simply has no provision for them at all. What it does, it does simply and reliably, which is why so many files still exist.

Mostly three places. Digital camcorders and early digital cameras, which recorded AVI to tape or memory card through the 2000s. Screen recorders and capture cards, which wrote it because it was easy to append to while recording. And the download culture of the same era, where DivX and Xvid inside AVI was the standard way video moved around.

That means an AVI in someone’s possession is usually old and usually irreplaceable — family video from a camcorder, a recording of something that no longer exists. The value is in the content rather than the format, which shapes what should be done with it.

AVI carries whatever codec the recorder used, and the ones from its era are the problem: DivX, Xvid, Motion JPEG, Cinepak, Indeo, and a long tail of proprietary schemes from individual camera manufacturers. Modern operating systems ship decoders for almost none of them.

So a file opens, shows the right duration, and plays as a black rectangle or nothing at all. That is not damage. It is a missing decoder, and it is why the same file plays perfectly in VLC — which bundles its own decoders — and not in the system player.

Every AVI names its video codec with a four-character tag written into the header: `DIVX`, `XVID`, `MJPG`, `DX50`, `cvid` and so on. It is the fastest way to find out what you are actually holding, and any media information tool will read it in a second.

It is worth doing before troubleshooting anything else, because the answer determines the next step. A common tag means a converter will handle it directly; an obscure manufacturer tag may mean the original camera software is the only thing that ever read it properly.

AVI’s audio handling assumes a constant bitrate, because that was the norm when it was designed. Variable-bitrate audio can be stored, but the container has no honest way to describe it, and the result is the classic AVI failure: sound and picture in step at the beginning and visibly apart by the end.

The drift is progressive rather than sudden, which is how you tell it apart from a damaged file. It is also not repairable inside AVI in any satisfying way — the reliable fix is to convert to a container that was designed for variable bitrate, which is any of them made since.

No subtitle track worth the name, so subtitles for an AVI have always lived in a separate SRT file beside it. No chapters. No menus. No standard place to record the orientation, which is why a video shot sideways stays sideways and has to be physically rotated rather than flagged.

No B-frame support in the original design either — the bidirectional frames that modern codecs rely on for compression efficiency — which is why unofficial extensions exist to squeeze H.264 into AVI and why those files play inconsistently. If an AVI is behaving strangely with a modern codec inside it, that mismatch is usually the reason.

AVI writes a small header before every chunk of data, and on video with many small frames that adds up to a meaningful fraction of the file. The same content in a modern container is smaller before any re-encoding has taken place, purely from bookkeeping.

Combined with codecs that are twenty years behind, an old AVI is frequently several times larger than an equivalent-looking MP4. Converting a camcorder archive is one of the rare cases where a file gets dramatically smaller and visibly better on the same screen, because the modern codec is doing more with fewer bits.

MP4 with H.264 is the destination for anything meant to be kept, watched or shared: it plays on every device made this century, streams over a network, and holds the orientation flag AVI lacks. Choose a quality setting on the generous side, because the source has already been through one generation of a weaker codec and there is nothing to gain from being frugal with the second.

Keep the original AVI if it is the only copy of something irreplaceable. Conversion is lossy, storage is cheap, and a camcorder tape from 2004 is not going to be re-recorded. Convert for watching and keep the original for keeping.

Video files are large and personal in a way that makes upload-based conversion a poor bargain — the AVIs people convert are overwhelmingly home footage, and a two-gigabyte upload is slow enough that most services impose a size limit that rules them out anyway.

The browser cannot do it either, which is why there is no AVI conversion page here. DivX, Xvid, Cinepak, Indeo and the camera-maker schemes predate WebCodecs and no browser exposes a decoder for any of them, so unlike a modern container an AVI cannot be read on the visitor’s own machine at all. The pairs are declared and waiting on a server-side ffmpeg build. Until that lands, VLC and HandBrake on the desktop are the answer: both carry their own decoders, and neither sends the footage anywhere.