Compress AVI videos

AVI: the Windows video format of the 1990s

AVI (Audio Video Interleave) was introduced by Microsoft in 1992 with Video for Windows. It is built on RIFF, the same chunk structure as WAV audio files, and simply interleaves pieces of audio and video. AVI itself does not compress anything; the codec inside does. Over the years AVI files carried uncompressed video, Cinepak and Indeo, DV from camcorders, Motion JPEG from early digital cameras and, most famously, DivX and Xvid (MPEG-4 Part 2) for movies shared in the 2000s.

You mostly meet AVI today in old personal archives, exports from older screen recorders and CCTV systems, digitized VHS tapes and footage from compact cameras made before about 2010.

Why AVI files are often huge

Many AVI files use codecs designed for editing or simple hardware, not for small size. DV uses 25 Mbps for the picture alone, close to 190 MB per minute before the audio is added, and Motion JPEG stores every frame as a separate JPEG with no prediction between frames. Even DivX and Xvid, efficient for their time, need noticeably more bitrate than H.264 for the same picture quality.

This is the good news: an AVI usually shrinks dramatically when it is re-encoded to H.264 in an MP4 file, often with no visible loss, because the old codec wasted so many bits. The original AVI cannot be compressed much while staying AVI.

What works in the browser

Browsers cannot play AVI files, and the in-browser reader the tool relies on handles modern containers such as MP4, MOV, MKV and WebM. The tool recognizes an AVI file but cannot read it, so it shows a clear message instead of a broken result. The way round this takes two steps:

1. Convert the AVI to MP4 on your computer with free desktop software such as HandBrake or VLC.
2. Bring the MP4 back here to reach a specific target size or to fine-tune the result.

The second step gives you an H.264 and AAC file that any phone, TV or browser can play. Useful settings for typical AVI sources once they are in MP4:

• Digitized VHS or DV camcorder footage: set Maximum resolution to Keep original (usually 720 × 576 or 720 × 480 anyway), High quality. Upscaling old video does not add detail.
• CCTV or dashcam clips: frame rate cap 24 or 30, Smallest file; the picture is mostly static.
• Old screen recordings: Balanced; sharp text needs a bit more bitrate than camera footage.

If you already convert with HandBrake, its own quality settings are often enough; the browser tool is most useful when the file has to fit a fixed limit such as 25 MB for email.

Typical problems with AVI

• Interlaced picture. Video from TV and camcorders is often interlaced and shows comb-like lines on moving edges. Desktop converters with a deinterlace filter handle this better than a quick re-encode.
• Audio out of sync. Some old AVI files have variable-bitrate MP3 audio, which the format handles badly; players have always worked around it, converters sometimes do not.
• The 2 GB barrier. Many programs could not write AVI files larger than 2 GB, and the RIFF structure itself stops at 4 GB; the OpenDML extension removed the limit, but some old tools still split long recordings into several files.
• Odd aspect ratio. DV and VHS captures have non-square pixels, so 720 × 576 is meant to be shown at 4:3 or 16:9. Check the preview before you delete the original.

AVI questions

Related pages

See the video compressor for all video formats, MKV for another container popular with movie files, and how it works for the browser technology behind the tool.