Free online MK3D to XVID converter

Some older media players, editors, and playback devices accept Xvid-encoded AVI files but do not recognize Matroska 3D files. Converting an MK3D file requires decoding its video and re-encoding it with the Xvid codec, usually inside an AVI container.

What the MK3D format is

MK3D is a filename extension for a Matroska container carrying stereoscopic 3D video. It can store video, multiple audio tracks, subtitles, chapters, and Matroska 3D metadata describing layouts such as side-by-side, top-and-bottom, or separate left- and right-eye streams.

MK3D files commonly come from 3D Blu-ray backups and Matroska muxing tools. Applications such as MakeMKV and MKVToolNix may create files using this extension, while VLC, MPC-HC, Kodi, and other Matroska-capable players can often open them. The extension identifies the container; the actual video may use H.264/AVC, MVC, or another codec.

What the Xvid format is

Xvid is an MPEG-4 Part 2 video codec, not a container format. Xvid video is commonly stored in an AVI file with an .avi extension, alongside MP3 audio for compatibility with older hardware and software.

Xvid is useful when a device specifically supports MPEG-4 Part 2 AVI files, or when a smaller file is needed than an equivalent high-bitrate source. It is older and less efficient than H.264, H.265, and modern AV1 encoding, so converting to Xvid can reduce compatibility with newer streaming and editing workflows.

How to convert MK3D to Xvid

FFmpeg is the most practical desktop tool because it can read Matroska containers and encode Xvid through the libxvid encoder. Install a build that includes both libxvid and libmp3lame, then inspect the source:

ffprobe input.mk3d

For a conventional Xvid AVI with MP3 audio, run:

ffmpeg -i input.mk3d -map 0:v:0 -map 0:a:0? -c:v libxvid -q:v 4 -c:a libmp3lame -b:a 192k -tag:v XVID output.avi

The -q:v 4 setting uses quality-based video encoding; lower values generally produce better quality and larger files. Use -q:v 3 or -q:v 2 for higher quality, or specify a target bitrate such as -b:v 1500k when the playback device requires a predictable size.

If the source contains several audio tracks, list them with ffprobe and select the desired one explicitly, for example:

ffmpeg -i input.mk3d -map 0:v:0 -map 0:a:1 -c:v libxvid -q:v 4 -c:a libmp3lame -b:a 192k output.avi

On Windows, macOS, or Linux, a graphical FFmpeg front end can expose the same settings: choose the MK3D file, set the video codec to Xvid, set the container to AVI, select MP3 audio, and start the encode. On 101convert.com, you can convert the file online for free without installing software; upload the MK3D file, choose Xvid or AVI if offered, and download the result.

Quality, compatibility, and 3D limitations

Xvid does not preserve Matroska’s 3D metadata. A side-by-side or top-and-bottom source may remain visually arranged in one frame, but the resulting AVI normally will not carry the information that tells a 3D player how to interpret it. MVC-based 3D video may also fail to decode or may be reduced to one eye, depending on the FFmpeg build and source stream.

For reliable 3D playback, first check the decoded output and confirm that both eyes are present. If the target device expects ordinary 2D video, use a deliberate conversion filter rather than assuming the device will interpret the stereo layout. Some older AVI players also reject variable frame rate, unusual pixel formats, or unsupported audio, so standardizing to stereo MP3 and a common pixel format may be necessary:

ffmpeg -i input.mk3d -map 0:v:0 -map 0:a:0? -vf format=yuv420p -c:v libxvid -q:v 4 -c:a libmp3lame -ac 2 -b:a 192k output.avi