Convert AVI to MP4 on Mac (and Open Old AVI Files)

Smol reads AVI as input. It cannot write AVI, so the direction is always out of AVI, never into it.

1. Open Smol and drag the AVI file (or a whole folder of them) into the window.
2. Choose Convert and set the output to MP4. MOV, WebM and MKV are the other video choices.
3. Run it. By default the result lands beside the original with a suffix, and the original is untouched.

In my test, a 607,376 byte AVI became a 317,308 byte MP4 (47.8% smaller), with video and audio intact. That is one synthetic test clip, not a promise for your footage. Old AVIs are often large because they used inefficient codecs, so a drop in size is common, but a re-encode can also make a file larger, and Smol will tell you either way.

Install ffmpeg once with Homebrew, then convert:

brew install ffmpeg
ffmpeg -i holiday.avi -c:v h264_videotoolbox -b:v 4M -c:a aac holiday.mp4

The h264_videotoolbox encoder uses your Mac’s hardware; raise -b:v for higher quality and a bigger file. For a folder of AVIs, wrap it in a shell loop:

for f in *.avi; do ffmpeg -i "$f" -c:v h264_videotoolbox -b:v 4M -c:a aac "${f%.avi}.mp4"; done

This works on every AVI that ffmpeg can decode, which is nearly all of them. VLC can also convert through File > Convert / Stream, but the menu is clumsy. See our VLC alternatives for Mac if you only want to play the files.

QuickTime Player plays only the codecs macOS includes. AVI is a wrapper, and many AVI files carry DivX, Xvid or other older video that macOS does not decode. I ran Apple’s command-line converter on my test AVI:

avconvert --preset PresetPassthrough --source src.avi --output out.mov
avconvert:  unable to read src.avi

The same tool read an MP4 and a camcorder-style transport stream without trouble, so the failure was the AVI itself. You will not fix it by renaming the extension to .mov. Convert it to MP4 instead. If the AVI does play, the codec inside happens to be one macOS supports, and you can export it from QuickTime as usual.

Same steps, different output. In Smol choose MOV instead of MP4. With ffmpeg, change the extension and keep the codecs Apple apps like:

ffmpeg -i clip.avi -c:v h264_videotoolbox -b:v 4M -c:a aac clip.mov

Pick MOV when the file is headed for Final Cut Pro or iMovie. Pick MP4 for everything else, including sharing, because MP4 plays on Windows, Android, TVs and the web. Going the other way is covered in converting MOV to MP4.

For one old AVI, ffmpeg is free and gives you full control, and VLC will play the file without converting it at all. Smol adds nothing you need for a single clip if you are comfortable in Terminal.

Smol earns its price on a folder of old camcorder or screen-capture AVIs, a Finder right-click instead of a command, and when you would rather not upload family video to an online converter. It is $29 once for one Mac: see Smol. For other formats, compare the best file converters for Mac, and to shrink the result afterward read the best video compressor for Mac.