http://trac.ffmpeg.org/wiki/Encode/MPEG-4

MPEG-4 Encoding Guide

​MPEG-4 Part 2, aka MPEG-4, Xvid, and DivX, is a video codec that was most widely used before the wide adoption of H.264. FFmpeg has two encoders to output MPEG-4 video. The external encoding library libxvid:

ffmpeg -i input.avi -c:v libxvid output.avi

...and the native encoder mpeg4:

ffmpeg -i input.avi -c:v mpeg4 -vtag xvid output.avi

The native encoder has the advantage of not requiring an external library. Both encoders should provide a similar output, but for lower bitrates/quality (e.g. 1000 kBit/s for 720p content), libxvid will deliver better quality than mpeg4.

The default FourCC stored in an MPEG-4-coded file will be FMP4. If you want a different FourCC, use the -vtag option. E.g., -vtag xvid will force the FourCC xvid to be stored as the video FourCC rather than the default.

Note: This guide uses the AVI container files for the examples, as the most common usage of XviD video is currently for older hardware devices. See the H.264 and AAC encoding guides if you are using modern devices. These codecs will offer better compression efficiency; using modern containers like MP4 or MKV is also recommended.

Variable Bit Rate with -qscale

You can select a video quality level with -qscale:v n (or the alias -q:v n), where n is a number from 1-31, with 1 being highest quality/largest filesize and 31 being the lowest quality/smallest filesize. This is a variable bit rate mode, roughly analogous to using -qp (constant QP [quantization parameter]) with x264. Most of the time this should be the preferred method.

You can select an audio quality level with -qscale:a (or the alias -q:a). The value changes depending on the audio encoder. Since this guide uses libmp3lame see the MP3 Encoding Guide for examples and more information.

Example:

ffmpeg -i input.avi -c:v mpeg4 -vtag xvid -qscale:v 3 -c:a libmp3lame -qscale:a 4 output.avi

There is rarely a need to use -qscale:v 1. Note that if you choose it, libxvid will take much more space than the same video compressed with the native mpeg4 encoder.

Constant Bit Rate

You can target a bitrate with -b:v. This is best used with two-pass encoding. Adapting an example from the x264 encoding guide: your video is 10 minutes (600 seconds) long and an output of 50 MB is desired. Since bitrate = file size / duration:

(50 MB * 8192 [converts MB to kilobits]) / 600 seconds = ~683 kilobits/s total bitrate
683k - 128k (desired audio bitrate) = 555k video bitrate

Two-pass example

ffmpeg -y -i input.avi -c:v mpeg4 -vtag xvid -b:v 555k -pass 1 -an -f avi /dev/null
ffmpeg -i input.avi -c:v mpeg4 -vtag xvid -b:v 555k -pass 2 -c:a libmp3lame -b:a 128k output.avi

Note: Windows users should use NUL instead of /dev/null.