Free online KUX to WEBM converter

WebM is required by some browsers, HTML5 video players, editing workflows, and web publishing systems, while KUX files are commonly downloaded from Youku. Conversion is necessary because changing the filename extension does not change the KUX container or its codecs.

What the KUX format is

KUX is a proprietary video container associated primarily with Youku downloads and desktop playback software. You may encounter it after saving offline video from Youku, especially when the download tool stores video in segmented or protected form.

KUX files can contain codecs that WebM does not normally support, such as H.264 video and AAC audio. Some KUX files are also encrypted or use Youku-specific structures, so a general-purpose converter may fail to read them.

What the WEBM format is

WebM is an open media container designed for web playback. Standard WebM files normally contain VP8, VP9, or AV1 video with Vorbis or Opus audio.

WebM is useful for HTML5 video, browser-based players, lightweight sharing, and web pages because major browsers support it without proprietary plugins. VP9 and AV1 can provide smaller files than older codecs at comparable visual quality, although encoding takes longer and compatibility varies by device.

How to actually convert KUX to WEBM

Using FFmpeg

Install a current desktop build of FFmpeg, place the KUX file in an accessible folder, and check whether FFmpeg can read it:

ffprobe input.kux

If FFprobe displays a video and audio stream, encode the file directly to WebM with VP9 and Opus:

ffmpeg -i input.kux -c:v libvpx-vp9 -crf 32 -b:v 0 -c:a libopus -b:a 128k output.webm

Use a lower VP9 -crf value for higher quality and a larger file; values around 28–36 are a practical range. For faster encoding, add -deadline realtime or -deadline good, although the default quality and speed balance is usually better.

To preserve the original frame rate and avoid unnecessary metadata, a more explicit command is:

ffmpeg -i input.kux -map 0:v:0 -map 0:a:0? -c:v libvpx-vp9 -crf 32 -b:v 0 -c:a libopus -b:a 128k -f webm output.webm

Using a two-step conversion

If FFmpeg can read the KUX file but direct WebM encoding produces an error, first create an intermediate file with broadly supported codecs:

ffmpeg -i input.kux -c:v libx264 -preset medium -crf 18 -c:a aac -b:a 192k intermediate.mp4

Then encode the intermediate file as WebM:

ffmpeg -i intermediate.mp4 -c:v libvpx-vp9 -crf 32 -b:v 0 -c:a libopus -b:a 128k output.webm

With the built-in online converter on 101convert.com, you can upload the KUX file, select WEBM as the output format, and download the result without installing software. The file must be readable by the service; protected or damaged KUX files may be rejected.

Quality, compatibility, and limitations

Do not use -c copy for a normal KUX-to-WebM conversion unless the source already contains WebM-compatible VP8, VP9, or AV1 video and Vorbis or Opus audio. Copying H.264 or AAC streams into a file named .webm does not create a standards-compliant WebM file.

FFmpeg cannot convert KUX files that are encrypted, incomplete, or unsupported by its demuxers. In that case, obtain an unprotected MP4 or another standard export from the original Youku download or playback application; renaming the file and changing container options will not remove encryption.

Repeated transcoding reduces quality. If the source is already heavily compressed, use a moderate VP9 quality setting such as -crf 30 to -crf 34, inspect the result, and avoid converting the same material through several intermediate formats.