Free online KUX to MP4 converter

Youku offline downloads stored as .kux often cannot be opened by editors, TVs, phones, or ordinary media players. An MP4 copy is useful only when the source is readable and you are authorized to export or convert it outside the Youku application.

What KUX files are

KUX is a proprietary file format associated with Youku, a Chinese video platform. The Youku desktop client may store offline downloads or cached video as KUX files for playback through the Youku application rather than as standard media files.

A KUX file can include Youku-specific metadata and may be encrypted or tied to the account and client that downloaded it. It is therefore not reliably readable by VLC, video editors, or general-purpose converters. Renaming video.kux to video.mp4 changes only the filename extension; it does not convert, decrypt, or repair the file.

What MP4 files are

MP4 is the MPEG-4 Part 14 media container format. It can hold video, audio, subtitles, chapters, and metadata, and it is supported by most operating systems, browsers, TVs, mobile devices, media servers, and editing applications.

For broad playback compatibility, an MP4 file should normally use H.264/AVC video and AAC audio. MP4 can also contain H.265/HEVC, but HEVC playback and editing support varies more widely, particularly on older devices and browsers.

Convert a readable KUX file

1. Confirm that the complete KUX download still plays in the Youku client and under the account that downloaded it. Do not use a partial download, a small cache fragment, or a file that Youku is still writing.
2. Open the KUX-to-MP4 converter on 101convert.com, select the .kux file, and choose MP4 if it is not already selected.
3. Start conversion, then download the resulting .mp4 file. The service can be used for free without installing software.
4. Test the MP4 in a media player before deleting the KUX source. Check both video and audio, and import a short section into the intended editor if editing is the goal.

If conversion reports encryption, DRM, an unsupported input, or an invalid file, use Youku's official download, export, or transcode option if one is available for that video. Availability depends on the Youku client, account permissions, regional rights, and the video's licensing restrictions.

A generic converter cannot remove DRM or account-linked offline-download protection. A KUX file that plays only inside Youku may have no usable MP4 conversion path without an authorized unprotected source.

Check readable files with FFmpeg

FFmpeg is useful when a KUX file is unprotected and its streams can be identified, but it does not decrypt protected Youku downloads. In a terminal opened in the file's folder, inspect the input with:

ffmpeg -i input.kux

If FFmpeg recognizes compatible video and audio streams, first try remuxing them into MP4 without recompression:

ffmpeg -i input.kux -map 0:v:0 -map 0:a? -c copy -movflags +faststart output.mp4

-c copy preserves the encoded streams and avoids quality loss, but it works only when FFmpeg can read the input and the source codecs are valid in MP4. A successful remux is not necessarily compatible with every target device.

For a conventional H.264/AAC MP4, re-encode instead:

ffmpeg -i input.kux -map 0:v:0 -map 0:a? -c:v libx264 -crf 20 -preset medium -pix_fmt yuv420p -c:a aac -b:a 192k -movflags +faststart output.mp4

Lower -crf values produce larger files with less additional compression; values from 18 to 23 are common. Re-encoding takes longer and cannot improve a low-quality offline download.

Quality and compatibility limits

Choose the highest quality available before downloading in Youku. Converting an already compressed offline copy adds quality loss when video is re-encoded, while stream copying does not.

Output size depends on the source codec, resolution, frame rate, and encoding settings. A re-encoded MP4 can be smaller or larger than the KUX file; smaller size does not indicate better conversion quality.

Keep the original KUX file until the MP4 has been tested. If the video has missing audio, failed playback, or incorrect duration after conversion, the source may be incomplete, protected, or use streams that require a different authorized export.