Convert AVI to MP4

Convert AVI videos to MP4 (H.264) directly in your browser. Free and private, with no upload, no size limit and no watermark.

🌐 Español

Drop your file here (.avi)

🔒 Private by design: your files are processed locally in your browser and never uploaded to any server.

Written and maintained by Last updated

AVI is a box, and the codec inside it is the real problem

AVI (Audio Video Interleave) is a container. It records where the video frames and audio samples sit in the file and very little else; what decides whether the file plays is the codec used for the streams it wraps. AVI files from the DivX and XviD years almost always hold MPEG-4 Part 2 video alongside MP3 or AC-3 audio, and older ones can hold Cinepak, Indeo or Microsoft Video 1, decoders that shipped with a desktop operating system two decades ago and were quietly dropped along the way.

That combination is why an AVI can play perfectly on the machine that produced it and do nothing at all on a phone, a smart TV or a browser tab. Converting to MP4 replaces both halves at once: the wrapper becomes the one every player expects, and the streams are re-encoded to H.264 video and AAC audio, the pairing hardware decoders have been built around for well over a decade.

What the Quality select is really setting

The three choices under Quality map to a single number in the encode command, libx264’s Constant Rate Factor:

CRF targets a perceived quality level rather than a file size, so a lower number means better quality and the resulting size depends entirely on how hard the footage is to compress. A static screen recording at CRF 23 can land at a fraction of the source; a grainy tape transfer at CRF 18 can land above it. All three settings use the fast x264 preset, which gives up a little compression efficiency in exchange for encode time, and all three write the MP4 with +faststart, so the file’s index sits at the front and a browser can start playing before the download completes.

Putting an AVI through it

  1. Drop the AVI onto the box above, or use Choose a file. The drop zone accepts .avi and nothing else, so a file with any other extension is refused with a message naming what it will take.
  2. Set Quality, or leave it on Balanced.
  3. Click Convert AVI to MP4 and watch the percentage. The first run in a fresh page load also has to fetch and start the WebAssembly FFmpeg core, so the counter can sit at zero for a while before it starts moving.
  4. Download the result. The name is your original with the extension swapped, so holiday-2004.avi comes back as holiday-2004.mp4.

The audio is re-encoded whatever you choose

Quality only touches the video stream. Audio is always encoded to AAC at 128 kbps regardless of the setting, which is a sensible default for speech and camcorder sound and a slightly stingy one for music. It is worth knowing if the AVI is a concert recording or a ripped music video, because picking High quality will not buy the soundtrack anything at all.

The command also sets no explicit stream mapping, so FFmpeg’s default selection takes one video stream and one audio stream. Nearly every AVI holds exactly that, but a rare multi-language capture will come out with a single audio track. If the sound was all you wanted, converting first and then running the MP4 through MP4 to MP3 works, at the cost of a second lossy pass; what that second pass costs you is laid out in the guide on why re-encoding loses quality.

There is no stream-copy shortcut for AVI

MKV to MP4 attempts a stream copy before it re-encodes anything, because an MKV usually already holds H.264 or HEVC video that an MP4 container can carry across untouched. AVI rarely does. Its typical payload is MPEG-4 Part 2 with MP3 audio, and while that can technically be placed inside an MP4, the result is a file plenty of phones and browsers still refuse to play, which defeats the point of converting in the first place. So this page always re-encodes, and a full re-encode on a single-threaded WebAssembly FFmpeg build is genuinely slow work: minutes for a long clip, running on your own CPU, with the tab left open.

In practice the thing that catches people out is expecting AVI to behave like MKV here. It cannot, and a converter that promised an instant remux for AVI would mostly be producing files that still do not play.

What the MP4 unlocks afterwards

The output is a plain H.264 MP4, which is exactly what the tools that refuse an AVI are waiting for. Compress Video accepts .mp4 only, with presets aimed at email, WhatsApp and Discord attachment limits. Trim Video accepts .mp4, .mov, .m4v and .webm, so a converted clip can be cut down without a second full encode. And if MP4 was not the target at all, the Video Converter hub takes an AVI directly and offers MP4, WebM or GIF, though its MP4 path runs one fixed quality level with no dial to turn.

Frequently asked questions

Why did my MP4 come out bigger than the AVI it was made from?

Re-encoding cannot see how the original was compressed. It starts from the decoded frames and compresses them again to hit the quality target you picked, and grainy, noisy or already-low-bitrate footage is expensive to encode at CRF 18, so the result can genuinely be larger than a heavily compressed DivX source. The finished screen prints the before and after sizes and labels the change as smaller or larger, so you see it straight away. If the file grew, start over on Balanced or Smaller file, remembering that clicking Process another empties the drop zone and you will need to pick the AVI again.

Does the conversion need an internet connection?

The first conversion in a page load does. The WebAssembly build of FFmpeg is roughly 31 MB and is fetched from a public CDN at that moment rather than shipped with the page, so it has to download once. After that it stays loaded in the tab, and further conversions in the same tab run without touching the network again.

Can I queue up several AVI files and convert them in one go?

No, this page handles one file at a time. Dropping a second file while the first is still waiting replaces it instead of adding it to a list, and the drop zone ignores new files entirely once a conversion is running or has finished. The pattern is convert one, download it, click Process another, then start the next.

Does converting change the resolution or the frame rate?

No. The encode command carries no scaling filter and no frame-rate argument, so the MP4 keeps the source AVI's pixel dimensions and timing exactly as they were. Only the codecs and the compression level change, which is why a 640x480 clip from an old camcorder stays 640x480 rather than being quietly upscaled to look more modern.

What happens to the AVI file sitting on my disk?

Nothing touches it. The file you pick is read into the page's memory and copied into FFmpeg's own in-browser virtual filesystem, the MP4 is built there, and what you download is a brand new file. A web page cannot overwrite a file you selected through a file picker, and this one never tries, so the AVI stays exactly where it was.

The bar reached 100% and nothing downloaded. What is happening?

The percentage comes from FFmpeg's own progress events during the encode, and the finished file still has to be read back out of the virtual filesystem and turned into a download link afterwards, which takes a moment on a long clip. If you get the generic failure message instead, the real FFmpeg error was written to the browser console, so opening the developer tools console is the quickest way to find out which stream it could not handle.

Related tools