Free online KUX to FLAC converter

A KUX file usually comes from the Youku desktop client and may be encrypted, while FLAC is an audio-only format used by music players, editors, and archival workflows. The conversion therefore requires extracting the audio from a playable KUX source, not simply changing the filename extension.

What the KUX format is

KUX is a proprietary container associated mainly with downloaded Youku video. Youku’s desktop software creates KUX files for offline playback, and the files can contain video, audio, metadata, and platform-specific encryption or protection.

A KUX file is not normally a standalone audio file. Its audio stream may be AAC, MP3, or another codec, depending on the source and download version. Some KUX files are readable by FFmpeg, but encrypted or newer variants may require playback or export through the Youku software before conversion.

What the FLAC format is

FLAC stores PCM audio with lossless compression. It reduces the size of uncompressed WAV or AIFF audio without discarding samples, supports metadata such as artist and album tags, and is supported by many desktop players, audio editors, media servers, and archival tools.

FLAC does not preserve video. Converting a KUX file to FLAC keeps only one selected audio stream and removes the picture, subtitles, and video metadata.

How to convert KUX to FLAC

Try FFmpeg first

Install a current FFmpeg build, place the KUX file in a convenient folder, and inspect its streams:

ffprobe -i input.kux

If FFmpeg recognizes the file and lists an audio stream, extract the first audio stream as FLAC:

ffmpeg -i input.kux -map 0:a:0 -vn -c:a flac output.flac

-map 0:a:0 selects the first audio stream, -vn excludes video, and -c:a flac encodes the decoded audio in the FLAC format. To preserve separate audio streams, inspect the ffprobe output and run the command again with the required stream index, such as -map 0:a:1.

If FFmpeg cannot read the KUX file

An error such as “Invalid data found when processing input” generally indicates that the KUX variant is encrypted or unsupported, rather than that the extension is wrong. Do not rename input.kux to input.flac; renaming changes no data.

Use the Youku client to play or export the download in a standard format if that option is available. Then extract the exported file’s audio with FFmpeg:

ffmpeg -i exported.mp4 -map 0:a:0 -vn -c:a flac output.flac

If the official client offers no export and the file is protected, a direct FLAC conversion may not be possible without an authorized tool that can decrypt that specific KUX version. Do not bypass access controls or DRM without the rights and permission to do so.

Use the built-in online converter

You can also try the free converter on 101convert.com: upload the KUX file, select FLAC as the output, start the conversion, and download the resulting file. This works only when the service supports the particular KUX variant and can access its audio stream; protected downloads may be rejected.

Quality and compatibility limitations

FLAC encoding is lossless from the decoded input, but it cannot restore quality already removed by a lossy KUX audio stream such as AAC or MP3. If the source is lossy, the FLAC file will usually be larger than the source while sounding the same.

For a fixed sample format, FFmpeg normally preserves the decoded sample rate and channel layout. Check the result with ffprobe output.flac. If you need a specific rate or channel count, add options such as -ar 44100 or -ac 2, but resampling or downmixing changes the audio data.