How to Convert AVI to MP4

Somewhere on an old hard drive there is a folder of AVI files — digitized family videos, footage from a camera you no longer own, downloads from the 2000s. They are enormous, and these days half your devices refuse to play them at all. One conversion fixes both problems at once.

The free AVI to MP4 tool re-encodes those files to H.264 video with AAC audio in an MP4 container. The tool's page says the result is typically 5 to 10 times smaller than the AVI — a 10-minute AVI that eats over a gigabyte can come down to something you can actually send someone — and it plays on phones, browsers, and TVs. It all runs in your browser, nothing is uploaded, and there is no watermark or account in the way.

What You Need Before You Start

Old footage deserves a little care before you touch it, so sort these out first:

The AVI file itself — and note the hard limit: it must be under 500 MB. Over that, the tool answers "File too large" and will not load it, no matter how patient you are.
A backup of the original, if the footage is irreplaceable — the tool's own FAQ recommends keeping the original AVI before converting family videos, and that is just good practice for anything you cannot re-shoot.
A desktop or laptop computer — this one really matters here. AVI files are big, old codecs are slow to decode, and the tool warns that files over 200 MB may be slow and could fail on phones with limited memory.
Time for the first run — the video engine is about 30 MB and downloads the first time you convert, then stays cached. Large AVIs also just take longer to encode; that is normal, not a problem.

Set expectations right: conversion preserves what is already in the AVI, it cannot restore detail the old compression already lost. What you gain is size and compatibility — a file you can actually share and play — and for old footage, that is the whole point.

How to Convert AVI to MP4, Step by Step

Step 1: Add your AVI file

Open the tool and drag your AVI onto the dropzone, or click the box to pick it from your device. Copied it already? Ctrl+V pastes it straight in. When it loads, the box turns into a file card showing the name and size, and a status line says it is ready and tells you to click convert. Check that size on the card before you go further, especially with old footage — if the number is over 500 MB, the tool will have refused the file outright with a "File too large" message instead of showing the card. Files over 200 MB do load, but with a warning that conversion may be slow and could fail on phones with limited memory, which is one more reason to do this on a computer.

Found the folder of old AVIs? Open AVI to MP4 in a new tab and drop the first one in — the file card tells you straight away whether it fits.

AVI to MP4 tool after an AVI file is added, with the file card, Ready status and enabled Convert button marked — **Figure 1 — Step 1.** The tool after an AVI file has been added. ❶ is the file card showing the file name and size — check it is under 500 MB. ❷ is the "Ready" status line telling you to click convert. ❸ is the Convert to MP4 button, now enabled.

Step 2: Click Convert to MP4

Click Convert to MP4 and the re-encode begins. On a first run, the status line pauses while the video engine (about 30 MB) downloads, and only then does the progress bar start moving — do not mistake that pause for a freeze. After that, the bar tracks the encode, and with AVI files this is the slow part: old codecs like DivX, Xvid, or raw uncompressed video take real work to decode, and the tool re-encodes at high quality (CRF 23) into H.264 plus AAC. Keep the tab in the foreground while it runs. And remember, your footage never leaves your device — the tool is FFmpeg running in WebAssembly inside your browser, which is especially nice when the file in question is a family video you would rather not upload to a stranger's server.

If the old footage is ready and the backup is safe, open the converter in a new tab and hit Convert to MP4 — then give it the time a big old file deserves.

Conversion in progress with the status line, progress bar partly full and disabled Convert button marked — **Figure 2 — Step 2.** Conversion in progress. ❶ is the status line (loading the video engine on a first run, then converting). ❷ is the progress bar filling as the AVI re-encodes. ❸ is the Convert to MP4 button, disabled while the job runs.

Step 3: Download your MP4

When the encode finishes, a Download link appears with the new file's size next to it. Click it and save the MP4 — and take a second to compare the two sizes, because this is the satisfying part. The tool's page says the MP4 typically lands 5 to 10 times smaller than the AVI, so a file that was too big to send anywhere is suddenly small enough to share, and it plays on phones, browsers, and TVs instead of only on the one old player that still understands AVI. Keep your original AVI backup somewhere safe, and the MP4 becomes the copy you actually use.

That folder of old footage is one conversion per file away from usable — open AVI to MP4 in a new tab and download the first MP4.

Conversion finished with the Download MP4 link and Done status marked — **Figure 3 — Step 3.** Conversion finished. ❶ is the Download link with the converted MP4's file size — compare it with the original AVI size from Step 1. ❷ is the status line confirming the job is done.

AVI vs MP4: Why Is the Old File So Huge?

AVI dates from 1992 — it was designed for CD-ROMs, which tells you most of what you need to know. It usually wraps outdated codecs like DivX or Xvid, or raw uncompressed video with no compression at all, which is how a 10-minute clip balloons past a gigabyte while the same video as MP4 fits in about 100 MB. MP4 with H.264 is more than twenty years of compression research newer: the same visual quality at a tenth of the size, with hardware decoding support in every phone and TV.

There is no reason to keep videos in AVI anymore except archival purism, as the tool's page itself admits — and even then, a high-quality MP4 is the more practical archive. The one thing conversion will not do is improve the picture. If the old AVI is blocky or soft, the MP4 will be blocky or soft too, just smaller and playable everywhere. That trade is still very much worth making.

Common Problems and Fixes

"File too large" on adding the file? The AVI is over 500 MB, and the tool will not load it. This is the catch to know about up front: the tool page's own example talks about a 1 GB AVI shrinking to 100–200 MB, but a 1 GB file cannot be loaded here at all because of the 500 MB cap. Check the file size before you start, and work with files under the cap.
Conversion taking forever? Normal for AVI. Old codecs decode slowly, big files encode slowly, and the first run also downloads the 30 MB engine. Use a desktop browser and let it run.
Failed on your phone? Phones have less memory, and the tool warns about exactly this for files over 200 MB. The page's own advice is that very large AVIs belong on a desktop — and within the 500 MB cap, that advice still holds.
Will the MP4 look better than the AVI? No. Conversion preserves existing quality but cannot add detail that is not there. What you gain is size and compatibility.
Nervous about converting the only copy of a family video? Keep a backup of the original AVI first, as the tool's FAQ recommends. The conversion itself never uploads anything — it runs locally in your browser — but a backup is about accidents in general, not about this tool.
Result still will not play somewhere? Unlikely after this conversion, since MP4/H.264 plays on phones, browsers, and TVs. Check the destination's own file-size limit before blaming the format.

Old footage is only useful if you can actually watch it and share it. Three clicks per file, and a folder that has been gathering dust since the 2000s turns into videos your family can open on their phones. Start with one file — the size difference alone will convince you to do the rest.

Open AVI to MP4 — Free

No sign-up, no upload, no watermark added to your export. Your files stay on your device.