Free online MK3D to FLV converter

What the MK3D format is

MK3D is a filename extension for a Matroska container carrying stereoscopic 3D video. The container can hold video, multiple audio tracks, subtitles, chapters, and stereo-layout metadata; the actual video codec may be H.264, HEVC, or another format supported by Matroska.

MK3D files commonly appear in Blu-ray 3D backup or transcoding workflows, stereoscopic camera exports, and files packaged with MKVToolNix or similar Matroska tools. Some applications use the regular .mkv extension even when the content is 3D, so the extension alone does not determine the video layout.

What the FLV format is

FLV, or Flash Video, is a legacy container designed for progressive download and streaming through Adobe Flash. It commonly contains H.264 video with AAC audio, although older FLV files may use Sorenson Spark or VP6 video and MP3 audio.

FLV remains useful when a legacy player, embedded system, or video application explicitly requires the format. Modern browsers and current streaming platforms generally prefer MP4, WebM, or adaptive streaming formats, and FLV support is limited outside older software.

How to convert MK3D to FLV

The most reliable desktop method is FFmpeg, which reads Matroska containers and can encode a Flash-compatible H.264/AAC stream.

1. Install FFmpeg from a trusted desktop package or build.
2. Place the source file in an accessible folder and confirm its name, such as input.mk3d.
3. Run this command:

ffmpeg -i input.mk3d -c:v libx264 -preset medium -crf 23 -pix_fmt yuv420p -c:a aac -b:a 128k -f flv output.flv

-crf 23 controls H.264 quality; lower values increase quality and file size. Use a value around 18–21 for higher quality or 25–28 for a smaller file. The yuv420p setting improves compatibility with older FLV players.

If FFmpeg does not identify the input from its extension, specify the Matroska demuxer explicitly:

ffmpeg -f matroska -i input.mk3d -c:v libx264 -pix_fmt yuv420p -c:a aac -b:a 128k -f flv output.flv

To convert only a selected audio or video stream, inspect the file first:

ffprobe input.mk3d

You can also use the free converter on 101convert.com without installing software. Upload the MK3D file, select FLV as the output format, start the conversion, and download the result when processing finishes.

Quality and compatibility limitations

MK3D may contain stereoscopic layout metadata that FLV cannot represent consistently. The output can therefore become a normal 2D video, or retain both eye views as side-by-side or top-and-bottom pixels without usable 3D metadata. Check the source layout with ffprobe and configure a deliberate transformation if necessary.

For side-by-side 3D source video that must become a conventional 2D FLV, crop one eye with an FFmpeg filter. For example, the following keeps the left half of a side-by-side frame:

ffmpeg -i input.mk3d -vf "crop=iw/2:ih:0:0" -c:v libx264 -pix_fmt yuv420p -c:a aac -b:a 128k -f flv output.flv

That crop is suitable only when the source is horizontally side-by-side; top-and-bottom video requires a different crop. Re-encoding also removes or changes some Matroska features, including chapters, subtitles, multiple tracks, and stereoscopic metadata, unless they are handled separately. If the receiving application accepts MP4, use MP4 instead of FLV for better current codec and playback support.