Free · no signup · never uploaded
AVI to MP4 Converter
Modernise an old AVI into an MP4 that still plays today.
Drop an AVI here or choose a file
or drop your file here.
AVI · converted in your browser, never uploaded
Runs in your browser — your file is never uploaded.
Short answer
To convert AVI to MP4, drop your file below and download the MP4. Old AVI files use codecs MP4 can’t carry, so this is a real re-encode to H.264 — and it still runs entirely in your browser, with nothing uploaded.
Why old AVI files stop playing
AVI is a container from 1992, and the files people still have tend to hold codecs of that era — DivX, Xvid, MPEG-4 Part 2, sometimes MJPEG. Modern phones, browsers and TVs simply dropped support for those. The footage itself is fine; it’s the codec that’s obsolete. Re-encoding to H.264 inside an MP4 puts the same content into a format that today’s devices decode in hardware.
What to expect from the result
Old AVI footage is usually low resolution by modern standards, and converting won’t add detail that was never captured. What you get is a file that plays — on a phone, in a browser, in an editor — and is typically a good deal smaller, since H.264 compresses far more efficiently than the codecs AVI carried. For archival footage, converting is the difference between a file you can watch and one you can’t.
Video formats to MP4 — what happens to your file
| Input format | Usual codecs inside | Conversion | Max file size |
|---|---|---|---|
| MOV (QuickTime) | H.264 + AAC | Rewrap (lossless) | 500 MB |
| MKV (Matroska) | H.264 + AAC | Rewrap (lossless) | 500 MB |
| WebM | VP8/VP9 + Opus | Re-encode to H.264 | 100 MB |
| AVI | MPEG-4/DivX | Re-encode to H.264 | 100 MB |
| GIF | Animated GIF | Re-encode to H.264 | 200 MB |
A rewrap copies the existing video and audio streams into an MP4 container untouched — it is lossless and finishes in seconds. A re-encode decodes and re-compresses, so it takes longer and loses a little quality. Size caps differ because everything runs on one thread in your browser.
How this conversion works
Everything happens locally in your browser with ffmpeg — here is exactly how:
- Load the engine. ffmpeg compiled to WebAssembly loads once from this site — your file is never uploaded.
- Decode the legacy codec. ffmpeg identifies and decodes whatever the AVI holds — commonly MPEG-4 Part 2, DivX, Xvid or MJPEG.
- Re-encode to H.264. frames are encoded with libx264 (veryfast preset, single thread) and audio is re-encoded to AAC for MP4 compatibility.
- Write the MP4. the result is muxed into an MP4 with the moov atom at the front so it plays immediately.
Assumptions
- Everything runs client-side; this is a full re-encode, so it is much slower than a rewrap.
- Conversion cannot improve on the quality the original AVI captured.
- Speed depends on your device, the clip’s length, and its resolution.
Last reviewed: July 19, 2026
Frequently asked questions
How do I convert an AVI to MP4?+
Drop your .avi in above and press convert. The old codec is decoded and re-encoded to H.264 in your browser — nothing is uploaded. Expect it to take a while; this is a real re-encode.
Are my videos uploaded to a server?+
No. Everything runs 100% in your browser using ffmpeg compiled to WebAssembly, so your file never leaves your device. Nothing is uploaded, stored, or sent anywhere — and once the converter has loaded it even works offline.
Why won’t my AVI play on my phone?+
AVI files usually carry older codecs like DivX, Xvid or MPEG-4 Part 2, which modern phones and browsers no longer support. Converting to H.264 in an MP4 fixes that.
Will converting improve the quality?+
No — conversion can’t add detail the original never captured. It makes the file playable on modern devices and usually smaller, but the picture can’t be better than the source.
Why is the first conversion slower?+
The first time, your browser downloads the ffmpeg engine (about 31 MB) and starts it up. That happens once — later conversions reuse the loaded engine and are much faster.
Why is the file-size limit lower here?+
Because every frame must be decoded and re-encoded rather than copied. Because it runs on a single thread inside your browser, speed depends on your device, the clip’s length, and its resolution.
Related tools
Video is processed entirely in your browser with ffmpeg and never uploaded.