Why convert AVI to MP4
AVI is a Windows format from the 1990s. It still opens in VLC, but it fails in many other places: the iPhone Photos app, QuickTime on a Mac, web browsers, most social networks and a lot of smart TVs. MP4 with H.264 video and AAC audio is the opposite. It is the most widely supported video format today. When you convert AVI to MP4, you get one file that plays on phones, computers, TVs and websites without installing extra codecs.
Re-encoding or a quick copy?
What happens depends on the codecs inside your AVI. The input panel shows them before you start.
- Xvid, DivX or Motion JPEG video (the most common case): the picture is re-encoded to H.264. This takes time, but the file usually becomes smaller. Old camera clips in Motion JPEG often shrink to a fifth of their size or less.
- H.264 video with AAC or MP3 audio: these codecs already fit MP4, so the streams are copied into the new container. This is almost instant and lossless.
Re-encoding cannot add detail that was not in the original. A 640x480 video from 2005 will still look like a 640x480 video. Keeping the original resolution is the best choice. Upscaling only makes the file bigger.
Settings that matter
For most people the defaults are right: H.264, original resolution, balanced quality. If you need a smaller file for email or messaging, choose 720p or 480p, or run the result through the video compressor. If you only need part of the video, convert first and then cut it with Trim Video. H.265 (HEVC) gives smaller files, but older computers and some browsers cannot play it, so use it only if you know your devices support it.
Fixing typical AVI problems
The AVI has no sound on the TV or phone. The audio is often AC-3 or another codec those devices do not decode. After you convert AVI to MP4, the sound is AAC, which every device plays. Sound slowly drifts out of sync. This happens with some old AVI files that used tricks to store B-frames. Converting usually fixes it because the new file gets clean timestamps. The file stops halfway. Downloads that were never finished are common in old AVI collections. The converter keeps the part that can be read.
Need the opposite direction for an old player? Use MP4 to AVI. Other targets are on the AVI converter page.