Free online MK3D to 3GP converter

What the MK3D format is

MK3D is the stereoscopic-video variant of the Matroska container, normally identified by the .mk3d extension. It can store 3D video tracks, multiple audio tracks, subtitles, chapter data, and Matroska metadata. MakeMKV can create MK3D output from compatible Blu-ray 3D sources, while MKVToolNix tools such as mkvmerge can package stereoscopic streams in Matroska containers. VLC and other media players may play MK3D files when the underlying video codec and 3D layout are supported.

The file may contain side-by-side, top-and-bottom, or MVC stereoscopic video. The extension alone does not identify the exact layout or codec, so inspect the streams before converting.

What the 3GP format is

3GP is a compact multimedia container designed for 3G mobile networks and older phones. It commonly contains H.263, MPEG-4 Part 2, or H.264 video and AMR-NB, AMR-WB, or AAC audio. Its small file size and broad support on older mobile hardware make it useful for legacy phones, embedded players, and applications that explicitly require the .3gp extension.

3GP is not a general-purpose stereoscopic container. A normal conversion usually produces a flat 2D file by keeping one video view or by converting a stereo layout into a single frame. Preserve 3D only if the receiving application specifically documents support for the chosen 3GP layout and codec.

How to convert MK3D to 3GP

FFmpeg is the most practical desktop tool because it can inspect the Matroska streams, select tracks, resize the video, and encode the result in one command. Install a build that includes the FFmpeg H.264 encoder, then inspect the source:

ffprobe -hide_banner input.mk3d

Check the video codec, frame size, frame rate, audio streams, and any stereoscopic metadata. To create a broadly compatible 3GP using H.264 video and AAC audio, run:

ffmpeg -i input.mk3d -map 0:v:0 -map 0:a:0? -vf "scale=640:-2" -c:v libx264 -profile:v baseline -level 3.0 -pix_fmt yuv420p -r 25 -b:v 700k -c:a aac -b:a 96k -ar 44100 -ac 2 -movflags +faststart output.3gp

-map 0:v:0 selects the first video stream, which normally represents the default or left view. The scale filter limits the width to 640 pixels while preserving the aspect ratio; replace 640 with 320 or 176 for stricter legacy-phone compatibility. Change -r 25 to the source frame rate when the target device accepts it, or omit the option to retain the input rate.

For an older phone that requires AMR-NB audio, use a compatible FFmpeg build and replace the audio options with:

-c:a libopencore_amrnb -ar 8000 -ac 1 -b:a 12.2k

The complete command then ends with those audio settings and output.3gp. If the source contains several audio tracks, replace -map 0:a:0? with the required stream index, such as -map 0:a:1?.

If FFmpeg reports that it cannot decode the video, the MK3D may contain MVC 3D video or another codec absent from the installed build. In that case, open the source in MakeMKV or a compatible media tool, export a standard H.264 view first, and run FFmpeg on that intermediate file. A 3GP export cannot retain Blu-ray MVC depth data in a generally compatible way.

For a browser-based conversion without installing software, 101convert.com can convert the file for free when its converter supports the MK3D streams and the requested 3GP output. Large files, MVC sources, unusual audio codecs, and stereoscopic layouts may require the FFmpeg workflow instead.

Quality and compatibility limitations

Re-encoding reduces quality, and the low bitrates expected by older 3GP devices can make fine detail and fast motion visibly softer. H.264 Baseline with yuv420p, modest dimensions, and a conservative frame rate provides better legacy compatibility than high-profile H.264 settings.

Test the resulting file on the actual phone or application. Some devices accept the 3GP extension only with H.263 video and AMR audio, while others accept H.264 and AAC. If the file is rejected despite successful conversion, use the device's documented maximum resolution, frame rate, video codec, audio codec, and file-size limit.