My AVI files still open, why should I convert them?

If you have a comprehensive player such as VLC on your computer, AVI probably opens without trouble. The problem starts when you move the file somewhere else: most smart televisions, phones, tablets and web upload forms either do not recognise AVI at all or cannot decode the codec inside it. Modern video editing programs can also struggle to import AVI. So the decision to convert is not about the file's situation today, it is about where you want to take it.

Will quality drop if I turn AVI into MP4?

Most likely yes, it will drop somewhat, and it is only fair to be honest about that. AVI files typically carry codecs such as DivX, Xvid or MPEG-4 Part 2; in practice the MP4 container does not carry these reliably, so the picture has to be re-encoded to H.264. Re-encoding is a lossy operation and some detail always goes. With good settings that loss is usually not noticeable to the naked eye, but technically it is there.

Why are my AVI files so large?

The reason is that both the container and the codec are old. The codecs of the AVI era lacked the advanced compression techniques used by H.264 and H.265; they needed far more data to achieve the same visual quality. The AVI container itself also carries overhead for every frame, and on long files that can add up to a considerable amount. That is why the same footage shrinks noticeably when compressed with a modern codec.

Should I convert my old family videos from AVI?

From an archival point of view the safest approach is to keep the original AVI files without deleting them and to produce an MP4 copy alongside each one. That way you have a file that opens on every device for daily use, while the source stays intact. If you delete the original, then when a better codec comes along later all you will have is the version that has been compressed once, and that loss does not come back. Because disk space is cheap today, holding on to both is a reasonable choice.

Why Did the AVI Container Age Out? What Does MP4 Bring?

The standard of an era

In the early 2000s, everyone who watched video on their computer had files with the .avi extension in their folders. Film sharing, camera recordings, screen captures — all of it was AVI. Today you only see that extension while rummaging through an old external drive.

AVI stands for Audio Video Interleave and was introduced by Microsoft in 1992. So this container was designed before the internet reached homes, at a time when it was not even clear what digital video was going to be. It answered the needs of that era perfectly well. The problem was that those needs changed completely.

What was AVI doing?

AVI, as the name suggests, stores audio and video by interleaving them: a chunk of picture, a chunk of sound, picture again. That structure was a sensible design for an era when data was read sequentially from disk and memory was very limited.

AVI is a container — it does not determine how the video inside it is compressed, it only carries it. In its own era, codecs such as Cinepak and Indeo circulated inside this container, and later DivX, Xvid and MPEG-4 Part 2. Those last three are the codecs of AVI's golden age, and if you have an AVI file in front of you today it most likely holds one of them.

The design limits that left AVI behind

AVI did not fall out of favour for a single reason. Several gaps in the container's structure hurt more and more as the video world moved on.

It cannot carry modern codecs properly. This is the most critical point. In practice AVI cannot reliably house today's codecs such as H.264 and H.265. Forced methods have been attempted in theory, but the result was always fragile and caused trouble in many players. If a container cannot carry the next generation of codecs, that container gets left behind.

It is not suited to variable bitrate audio. AVI was designed around a structure that assumes audio flows at a constant rate. When variable bitrate audio is used, a drift can develop between sound and picture that accumulates and grows as the minutes pass. Lip sync falling apart towards the end of a long film is the classic complaint of the AVI era.

Subtitle support is weak to the point of absence. Where MKV can carry eight subtitle tracks, AVI cannot put subtitles properly inside the container at all. That is why in that period subtitles were carried alongside the video as separate .srt files — the origin of a habit that persists to this day.

No chapter markers, rich metadata or multi-track management. Most of the organisational features a modern container offers are simply absent in AVI.

High overhead. AVI carries extra information for every piece of data, and on long files that turns into a considerable loss of space. Modern containers do the same job with far less excess.

The 4 GB limit problem. The original AVI structure carried serious limitations with large files. Extensions came along later, but compatibility always stayed rough.

What did MP4 bring?

MP4 was not designed to fill the gap AVI left behind, but directly for modern video needs.

Compatibility with modern codecs. MP4's real strength is that it carries H.264 and H.265 natively. Those codecs achieve the same visual quality with far less data than AVI-era codecs. The difference in between means a one-gigabyte AVI of the same film fits into a much smaller MP4.

Designed for streaming. MP4 can be arranged so that playback starts before the whole file has downloaded. AVI was not built with such a world in mind. This feature was decisive in making video over the internet the standard.

Universal device support. It is hard to find a device today that will not open MP4. Phones, televisions, consoles, browsers — all of them carry an H.264 decoder at hardware level. That means playback that is both fast and easy on the battery.

Proper audio/video sync. MP4's timing structure is built to cope with variable bitrate audio and variable frame rate video.

Subtitle and metadata support. Not as flexible as MKV, but incomparably more capable than AVI.

To be honest: this conversion is not free

When moving from MKV to MP4, changing the container is usually enough and no quality is lost. With AVI the situation is different.

Inside the AVI there is most likely DivX, Xvid or MPEG-4 Part 2. In practice the MP4 container does not carry these codecs reliably. Which means this: going from AVI to MP4 almost always requires a genuine re-encode. The picture is opened up, decoded frame by frame and compressed again with H.264.

Re-encoding is a lossy operation. Some detail always goes. When a good bitrate is chosen that loss is generally not noticeable to the naked eye, but technically it is there and it cannot be undone. It also takes time: on a long AVI file the process can run for many minutes.

It matters to approach this knowingly, because what you get in return is real: a smaller, modern file that opens on every device. Our AVI to MP4 converter performs this operation with sensible settings, but we still recommend keeping your original file rather than deleting it.

In summary

AVI was not a badly designed container; it was simply answering the questions of 1992. Streaming over the internet, video on a mobile phone, hardware decoding, multilingual audio and subtitles — none of these were on that design's agenda. MP4, by contrast, was made for exactly this world, and because it can carry modern codecs it produced files that are both smaller and more compatible.

If you have AVI files on your hands, moving them to MP4 is a sensible move — as long as you know that re-encoding has a cost, and you keep the originals to one side.