How to use it· 4 steps
- Drop your AVI file into the box above — it is read in place, not uploaded.
- Check the summary: it says whether the streams will be copied (seconds) or re-encoded (minutes), and why.
- Leave “Convert to” on MP4, choose an audio track if the file has several, and press Convert video.
- Download the MP4 when it appears. The first conversion also downloads the 32 MB engine.
Step by step
How to use the avi to mp4 converter
- 1Drop your AVI file into the box above — it is read in place, not uploaded.
- 2Check the summary: it says whether the streams will be copied (seconds) or re-encoded (minutes), and why.
- 3Leave “Convert to” on MP4, choose an audio track if the file has several, and press Convert video.
- 4Download the MP4 when it appears. The first conversion also downloads the 32 MB engine.
What is inside an AVI file
AVI is Microsoft's Audio Video Interleave format from 1992, one of the oldest video containers still in circulation. It predates modern codecs and has no proper way to store B-frame timestamps, variable frame rates or many of the features newer formats rely on. Most AVIs found today hold MPEG-4 Part 2 video — the codec behind DivX and Xvid — with MP3 audio; older camera and capture files may hold Motion JPEG, DV or even uncompressed video.
None of those video codecs play in a browser or on a phone, so AVI to MP4 almost always means re-encoding the video to H.264. The MP3 audio, which MP4 accepts, is copied unchanged.
Where AVI files come from
The AVI files people bring here are mostly a film or episode encoded with DivX or Xvid in the 2000s, a digital camera or camcorder of the same era, which often recorded Motion JPEG AVI, or a capture from a VHS digitiser, dashcam or an old screen recorder. In practice few of these can simply be copied, so plan for a real re-encode.
The converter does not guess. When you drop the file it reads the container's own description of its streams — codec, resolution, frame rate, audio tracks — and shows you, before you press anything, whether the MP4 will be a copy or a re-encode.
What happens to your AVI
The converter reads the AVI's streams and decides per stream. Xvid, DivX, Motion JPEG, DV and Windows Media video are re-encoded to H.264; in the rarer case of an AVI that already holds H.264, it is copied, with timestamps regenerated so the MP4 plays smoothly. MP3 or AAC audio is copied; PCM or ADPCM audio from cameras is re-encoded to AAC.
Interlaced material from camcorders and capture cards is detected and deinterlaced during the re-encode, which removes the comb-like stripes otherwise visible on movement.
How long AVI to MP4 takes
DivX and Xvid files are usually standard definition — around 640×360 to 720×576 — and SD encodes far faster than HD, even single-threaded: roughly real time on a recent computer, so a 45-minute episode takes something like 40 minutes to an hour.
Motion JPEG camera clips are short and usually 640×480 or 720p; a few minutes of footage converts in a few minutes. The output is often dramatically smaller, because Motion JPEG compresses every frame as a separate photo.
Problems you may run into
The browser cannot preview the AVI — browsers do not play AVI at all. The converter reads it with ffmpeg instead, so the conversion works even though there is no preview before it.
Audio drifts out of sync on a long file — some old AVIs were muxed with variable-bitrate MP3 that their own players handled loosely. If drift appears, choose 'Always re-encode', which rebuilds the audio timing as well.
The picture looks squashed or stretched — AVIs from DVDs often stored a pixel aspect ratio flag that some encoders ignored. The MP4 keeps the flag the AVI declares; if the source was already wrong, it stays wrong.
When not to convert AVI to MP4
If an old AVI plays fine in VLC and you only want to watch it, you do not need to convert it — VLC plays nearly everything.
If the AVI is uncompressed or Huffyuv capture footage meant for editing, keep the original as the master and convert a copy; re-encoding is the moment detail is lost.
Your AVI is never uploaded
Video is where uploading hurts most. A typical online converter makes you send the whole AVI — often hundreds of megabytes of family footage or work recordings — to its server before anything happens.
This converter skips all of that. ffmpeg, compiled to WebAssembly, runs inside this tab and reads your AVI straight from your disk; the MP4 is assembled in memory on your machine and handed to you as a download.
Only the engine travels: about 32 MB, fetched the first time you use any of the video tools and kept for the session. Disconnect afterwards and the conversion still runs.
Answers
Frequently asked questions
Why does AVI to MP4 have to re-encode?
Most AVIs hold Xvid, DivX or Motion JPEG video, which phones, browsers and modern editors do not support inside MP4. The video is converted to H.264, which they all do.
Will my old DivX film look worse?
Very slightly, as with any re-encode, but at the default quality the loss is difficult to see on SD material, and the file is often smaller than the original.
Can I convert AVI from my old digital camera?
Yes — Motion JPEG and PCM audio, the usual camera combination, both convert, and the MP4 is typically much smaller.
Is this AVI to MP4 converter safe for private videos?
Yes. Nothing is uploaded — the conversion runs locally in the page — so the video never leaves your computer or phone.
How large an AVI can I convert?
There is no artificial cap. The practical ceiling is the tab's memory, since the output is assembled there — comfortably a gigabyte or so on a typical computer, less on phones.