User Guide
Add your AVI file
Drag the .avi onto the drop zone or click to browse. The first run downloads the conversion engine (~30 MB, one time); after that it starts immediately. Files up to 300 MB are accepted.
Wait for the engine to report ready
A status line tells you when FFmpeg has loaded. Nothing leaves your device at any point — if you lose your connection after the engine has loaded, the conversion still finishes.
Press Convert
The video track is re-encoded to H.264 and the audio to AAC. There are no quality settings to choose: the tool uses the veryfast x264 preset at FFmpeg’s default quality (CRF 23), which is visually transparent for almost all AVI sources.
Watch the progress line
Expect roughly real-time speed on a modern laptop — a 5 minute clip takes about 5 minutes. Older machines and 1080p sources run slower. Keep the tab in the foreground; background tabs are throttled by the browser.
Download the MP4
The result plays natively in Safari, Chrome, Edge, Firefox, iOS, Android, PowerPoint and every major video editor. Check it opens before deleting the original.
If the conversion fails
Almost every failure is memory. Trim the clip first with the Video Trimmer or shrink it with the Video Compressor, then convert the smaller file.
About the AVI to MP4 Converter
AVI is a container Microsoft published in 1992 as part of the RIFF family — the same chunk-based structure still used by WAV. It predates the smartphone, the modern web browser and streaming video, and it shows. This tool rewrites an AVI into MP4 with H.264 video and AAC audio: the combination every browser, phone, TV and editing suite understands.
Why AVI files stop working
The problem is rarely the container itself. It is what people put inside it. Through the 2000s AVI was the default wrapper for DivX, Xvid, MJPEG, Cinepak and Indeo — codecs that needed a system-wide codec pack to decode. Browsers never shipped any of them, and no modern phone can play them. The MDN media container guide lists AVI as having essentially no native browser support, while MP4 is universal.
AVI also has structural limits that MP4 does not. The original RIFF index is 32-bit, which caps a strictly conformant file at 4 GB (the OpenDML extension raised this, but support is inconsistent — see the Microsoft AVI RIFF reference). AVI has no clean way to store variable frame rate video, and its B-frame handling was bolted on rather than designed in. MP4, standardised as ISO base media format, was built for all of it.
What actually happens to your file
This is a re-encode, not a rewrap. The tool decodes every frame and encodes it again as H.264 (ITU-T H.264), because the codec inside an AVI is almost never already H.264. That means there is a generation of quality loss. It is small — CRF 23 is FFmpeg’s default and is designed to be visually transparent — but it is real, and it is worth knowing before you delete the source.
| Property | Typical AVI source | MP4 output |
|---|---|---|
| Video codec | Xvid, DivX, MJPEG, Indeo | H.264 (AVC) |
| Audio codec | MP3, AC-3, PCM | AAC-LC |
| x264 preset | — | veryfast |
| Quality | — | CRF 23 (FFmpeg default) |
| Browser playback | None | All modern browsers |
| Practical size ceiling | 4 GB (32-bit index) | No practical limit |
What the output will weigh
File size follows one equation, and it is worth internalising because it explains every surprise:
size (MB) ≈ total bitrate (kbps) × duration (s) ÷ 8192
A 10 minute 720p clip encoded at roughly 2,000 kbps video plus 128 kbps audio comes out at (2128 × 600) ÷ 8192 ≈ 156 MB. Old Xvid AVIs were often encoded at high bitrates for their resolution, so the MP4 is frequently smaller than the source at the same visible quality — H.264 is roughly twice as efficient as Xvid at matched quality.
But not always. If your AVI was already heavily compressed, re-encoding can produce a larger file. If shrinking is the actual goal rather than compatibility, use the Video Compressor, which caps resolution as well as bitrate.
Privacy and limits
Everything runs through a WebAssembly build of FFmpeg that is downloaded into the browser tab on first use (about 30 MB, cached afterwards). The video itself is never transmitted — there is no upload step, no server-side queue and nothing to delete afterwards. Because a single-threaded WebAssembly module is capped at roughly 2 GB of addressable memory, files above 300 MB are rejected outright and anything over 60 MB shows a slow-work warning before it starts. A 300 MB AVI is roughly 25–40 minutes of standard definition footage, which covers most archive material people bring to this page.
Conversion speed is bounded by your CPU, not your connection. WebAssembly runs at roughly 50–80% of native FFmpeg speed, and this build is single-threaded, so a desktop with eight cores will not be eight times faster than a laptop.
Frequently Asked Questions
Will converting AVI to MP4 lose quality?
Yes, slightly. The codec inside an AVI is almost never already H.264, so the video has to be decoded and re-encoded. The tool uses CRF 23, FFmpeg’s default, which is chosen to be visually transparent — on normal viewing you will not see a difference, but it is one generation of loss. Keep your original until you have checked the result.
Why will my AVI not play on my phone or in a browser?
Because the codec inside it — usually Xvid or DivX — was never implemented by phones or browsers. On Windows a codec pack made it work; nowhere else did. Converting to H.264 in an MP4 container fixes this permanently.
Is there a file size limit?
Yes. Files over 300 MB are rejected, and anything over 60 MB shows a warning that it will be slow. The limit exists because a single-threaded WebAssembly module can address only about 2 GB of memory, and decoding needs several times the file size in working space.
Does the audio get converted too?
Yes. AVI audio is typically MP3, AC-3 or uncompressed PCM. All of it is re-encoded to AAC-LC, which is the audio codec MP4 players universally expect.
Does my AVI file get uploaded anywhere?
No. FFmpeg is compiled to WebAssembly and runs inside your browser tab. The file is read from disk into memory and the result is written back to disk. Nothing is transmitted, so there is nothing for us to store or delete.
Why is it slower than a desktop converter?
Two reasons: WebAssembly runs at roughly 50–80% of native speed, and this build is single-threaded so it uses one CPU core. Expect around real-time — a five minute clip takes about five minutes. Desktop FFmpeg uses every core and is several times faster.
Will subtitles or multiple audio tracks survive?
No. The conversion keeps the first video track and the first audio track. AVI rarely carries either extra, so this is seldom an issue — but if your file has a second language track, it will not be in the MP4.
Can I convert several AVI files at once?
Not in one pass — the tool handles one file at a time to stay inside the browser’s memory budget. Convert them one after another in the same tab; the engine stays loaded after the first run, so subsequent files start immediately.