MKVToolnix is a free, open-source toolkit for creating, modifying and examining Matroska media files on Windows, Linux and macOS. It works with the video, audio, subtitles and supporting information stored inside a media container, letting you assemble an MKV file from selected components or work on components already present. The package includes a graphical application and separate command-line utilities.
How MKVToolnix combines source files
The multiplexer brings streams from different source files into one Matroska file. You can combine video from an MP4 file with separate audio and SRT subtitles, or open an existing MKV and choose which of its tracks to copy into the output.
Adding and appending files perform different jobs in MKVToolnix. Added files contribute tracks that share the playback timeline, such as a soundtrack and its subtitles. Appended files follow the preceding content in sequence, which is the operation used to join consecutive parts. The graphical application can save source selections and track options as a configuration for later use.
Track names, flags and timing
Each track can have a name and language, and its position in the output track order can be changed. These details distinguish several soundtracks or subtitle sets within the same file. Default-track flags help a player choose a track alongside its own language preferences; forced-display flags can identify subtitles intended for foreign-language dialogue or onscreen text.
MKVToolnix also adjusts a track’s timestamps. A delay shifts that track against the others, while a timestamp scaling factor addresses a difference that grows over the running time.
Chapters
Chapter information gives a Matroska file named navigation points. MKVToolnix includes a chapter editor and can import chapter files when multiplexing. It can also generate chapter entries at regular intervals or where appended files begin, giving a joined recording navigation points based on its component parts.
Attachments
Attached files travel alongside the media tracks inside the Matroska container, with their own stored names, types and optional descriptions. When creating several output files through splitting, an attachment can accompany every part or only the first, rather than always being duplicated across the set.
File splitting
MKVToolnix can divide output by a target size, duration, specified timestamps or chapter boundaries. It can also retain selected time ranges while discarding the intervening material. Retained ranges may become separate files or be placed consecutively in one output.
Video splitting follows keyframes, the frames from which decoding can begin independently. As a result, a requested cut can land away from the exact timestamp or frame you entered.
Header editing
For changes to an existing file’s title, track name, language or default flag, the header editor modifies the Matroska properties. The accompanying mkvpropedit command-line utility writes such changes into the existing file without rebuilding the complete container. It can also replace chapter information, update tags and add, replace or remove attachments.
Changing a soundtrack’s language label can be handled as a property edit; selecting a different collection of media tracks belongs in the multiplexer.
Track extraction
The bundled mkvextract command-line utility separates components from a Matroska file. You select tracks by their identifiers and assign output filenames, and can also retrieve attachments, chapter information, tags and timestamps. Several types of extraction can run during the same pass through the input.
The stored track type determines the extracted format. Simple text subtitles become SRT files, ASS subtitles remain ASS, and FLAC audio is written as FLAC. Renaming an output extension does not choose a different conversion format. Chapters can be exported as XML, while attached files are recovered as their original contents.
Container inspection
The information tool shows the elements inside a Matroska file, including its tracks and codecs. The mkvinfo command-line utility can show a short summary or more detailed element information, including positions, sizes and track statistics.
