Convert · runs on your device

Convert AVI to MP4 in your browser

Drop your AVI files in and Video Mender re-encodes them to H.264 MP4 locally. Because AVI normally holds inefficient MPEG-4 Part 2 video, the MP4 is often half the size at the same visible quality.

Nothing is uploaded

Drop video files here

AVI

Choose video files to convert

Output format

Plays on effectively every browser, phone, TV and editor when it holds H.264 and AAC.

Video codec

The universal baseline. Hardware decoding exists on essentially every device sold since 2010.

How to size the output

A constant quality target. File sizes vary with how demanding the footage is.

Quality — 8 of 10

Resolution limit

Files smaller than the limit are left alone. Capping usually looks better than lowering the bitrate.

Frame rate

Audio

Fast rewrapCopy streams without re-encoding when the codecs already fit

Lossless and near-instant when it applies. It cannot reduce file size, so turn it off if you need a smaller file.

AVI files are usually old, and what is inside them is usually Xvid or DivX — MPEG-4 Part 2 video from the early 2000s. That codec needs roughly 60% more bits than H.264 for the same picture, which is why converting an AVI to MP4 so often produces a dramatically smaller file that looks no worse.

The conversion is a genuine re-encode. Every frame is decoded and compressed again, so it takes real time and involves a real quality decision. The default High quality preset is chosen so the H.264 output stays visually faithful to a source that was never pristine to begin with.

AVI also has a structural quirk worth knowing: its index sits at the end of the file. A truncated or partially downloaded AVI will often play in VLC but fail to convert cleanly, and Video Mender reports that as a failure rather than writing a broken MP4.

What this actually does to your file

Rewrap or re-encode

A minority of AVI files already contain H.264, and those can be rewrapped with no re-encode — the queue detects it and shows the fast badge. Xvid, DivX and Motion JPEG all require re-encoding, because MP4 has no reliable way to carry them.

Quality

This is a lossy re-encode on top of an already lossy source, so generation loss applies. At High quality the loss is hard to see; below Balanced, blocking in dark areas of old footage becomes obvious.

Old AVI footage is frequently interlaced. This converter does not deinterlace, so combing artefacts in the source will still be there afterwards.
Very old AVI files sometimes have variable frame rates that confuse MP4 timing. Setting an explicit frame rate fixes audio drift when it happens.
A re-encode cannot restore detail the original codec threw away. Converting will not make a 2003 DivX rip look modern.

How to use this page

Add your AVI files — a whole folder of them is fine.
Leave the output as MP4 with the High quality preset.
Optionally cap the resolution; most AVI sources are 480p or smaller, so leave it at source.
Start, then download each MP4 as it completes.

Where the result plays

Checked against the containers and codecs this page produces, stated conservatively.

Target	Works	Detail
Browsers	Yes	No browser plays AVI. Every browser plays H.264 MP4.
Phones and tablets	Yes	MP4 opens in the native gallery on both iOS and Android; AVI does not.
Video editors	Yes	MP4 imports cleanly. AVI import is inconsistent and often audio-only.
Legacy DVD players	No	Hardware built for DivX AVI may not accept H.264 MP4. Keep the original if that is your target.

The formats involved

Format	Read	Write	What it is
AVI	Yes	Yes	Microsoft Audio Video Interleave, from 1992. No native support for modern per-frame timing, B-frame handling is fragile, and the index sits at the end of the file.
MP4	Yes	Yes	The MPEG-4 Part 14 container. It is a wrapper, not a codec — an MP4 usually holds H.264 or HEVC video with AAC audio.
MKV	Yes	Yes	Matroska. The most permissive common container: it will carry almost any codec, plus multiple audio tracks, chapters and subtitles.
WebM	Yes	Yes	A deliberately restricted Matroska profile for the web: VP8/VP9/AV1 video with Opus or Vorbis audio, and nothing else.

Tested limits

Real ceilings measured on real devices, not a marketing claim of unlimited.

Limit	Desktop	iPhone / iPad	Why
File size — rewrap	1.3 GiB	384 MiB	Rewrapping never decodes a frame, so it tolerates far larger files.
File size — re-encode	1.0 GiB	256 MiB	Decoding holds uncompressed frames in memory, which is the real constraint.
Clip length — re-encode	4:00:00	30:00	Beyond this, progress reporting and memory behaviour stop being dependable.
Resolution — re-encode	4096 × 2160	1920 × 1080	A 4K frame is about 12 MB decoded, before the encoder allocates anything.

If something goes wrong

The conversion failed part way through.

The AVI is probably truncated — its index lives at the end of the file, so an incomplete download breaks conversion even though VLC may still play what is there. Re-obtain a complete copy.

The audio drifts out of sync.

A variable or misdeclared frame rate in the source is the usual cause. Set an explicit frame rate, typically 25 or 30 fps, and convert again.

The MP4 came out bigger than the AVI.

That points to an uncompressed or Motion JPEG source, or the Near-original preset on a low-bitrate file. Choose Balanced, or use target-size mode in the compressor.

Your video never leaves this device

This is a structural property of the application, not a policy we ask you to take on trust. There is no upload endpoint, no API route and no server action anywhere in the codebase, so there is nowhere for a video to be sent even by mistake.

Conversion runs in WebAssembly inside this browser tab. The engine is served from this domain and loaded only when you start a job.
The Content-Security-Policy restricts network connections to this origin, so the page is not permitted to contact a third party at all.
Nothing is stored. Close the tab and every queued file and finished output is gone — which is also why you should download results before you leave.
There are no accounts and no AI. Anonymous page views are counted — which page, roughly what country, what kind of device — and nothing whatsoever about your files.

The repository contains a test that fails the build if any media byte is passed to fetch, XMLHttpRequest, sendBeacon or a WebSocket. See the privacy page for how that is verified.

Questions

Why is the MP4 so much smaller than the AVI?

H.264 is far more efficient than the Xvid or DivX video inside a typical AVI. At matched visible quality it needs roughly 40% fewer bits, and the MP4 container carries less overhead than AVI too.

Does converting AVI to MP4 lose quality?

Yes, a little — it is a re-encode. Whether you can see the loss depends on the preset. High quality is chosen as the default because old AVI footage does not have detail to spare.

How long does an AVI conversion take?

Most AVI files are standard definition, which encodes quickly: expect well under real time for a 480p clip on a laptop. Progress shown in the queue is measured, never estimated.

Can I convert hundreds of AVI files at once?

Yes, and this is the common case for people digitising an old collection. The queue schedules by available memory and lets you download sequentially so nothing large is held in RAM unnecessarily.

Will the video be deinterlaced?

No. Deinterlacing is a separate decision with its own trade-offs and is not part of this converter. Interlacing artefacts present in the AVI will remain in the MP4.