MKVToolNix is a toolkit for creating, altering, and inspecting Matroska and WebM files. It merges video, audio, and subtitle tracks from different sources without re-encoding, and edits existing files in place.
MKVToolNix is a collection of programs that build and modify Matroska files, the container most commonly seen with an MKV extension. The core tool, mkvmerge, takes video, audio, and subtitle tracks from several source files and writes them into one container by copying the streams rather than decoding them, so nothing is lost and the process is quick. From there the same suite can split a file by size, duration, or timestamp, append parts back together, extract individual tracks, and inspect exactly how a file is structured. A graphical interface wraps the command line tools for day-to-day work.
MKVToolNix stays useful long after a file has been created, because most changes do not require rebuilding the container. A header editor rewrites track properties such as the track name, language, and default flag inside an existing file, and the same approach covers tags, chapters, and attachments. That matters for libraries where a subtitle track was labelled with the wrong language or an audio track needs to be the default choice. Chapter handling is equally thorough: chapters can be imported from other containers, written from a text file, edited directly, or generated automatically at fixed intervals. Kdenlive is the natural next step when a finished file needs actual editing rather than repackaging.
The advantage of MKVToolNix is precision without quality loss. Copying streams instead of transcoding means a remux takes seconds rather than hours, and the result is bit-for-bit identical to the source material, which matters when an archive or a delivery master is involved. Track-level control is the other strength: each stream can be given a name, a language, and default or forced flags, and audio can be offset by milliseconds to correct a sync problem without touching the picture. Splitting and appending support everything from keeping episodes at a fixed size to rebuilding one file from several parts. Because the command line tools accept scripted input, repetitive jobs such as relabelling a whole series can be automated, and the graphical interface covers the same ground for occasional tasks.
Key capabilities and highlights.
The mkvmerge program joins video, audio, subtitle, and chapter streams from several files into a single Matroska container by copying the data rather than re-encoding it. That lets a project combine a video file with better audio from another source or add subtitle tracks without spending hours on a transcode.
Every stream can be named, assigned a language, and marked as default, forced, or neither, so players pick the right audio and subtitle tracks automatically. Audio delay and stretch options correct sync drift, and track order can be rearranged to suit a player's preferences.
Output can be split by file size, by duration, or at specific timestamps, which suits burning to a fixed capacity or breaking a long recording into chapters. Existing files can be appended to build one continuous stream from separate parts, with chapter names generated automatically as each part joins.
MKVToolNix includes a chapter editor for creating entries, renaming them, and adjusting timestamps, with import and export for the common chapter file types found in other containers. Chapters can also be generated at fixed intervals or created whenever a file is appended, and the language applied to entries is configurable.
MKVToolNix can change properties of a finished file in place without remuxing it, using the same editing engine as the command line tool. Track names, languages, default flags, the overall file title, and other segment fields can be corrected, which is faster and safer than rebuilding a container just to fix a label.
Global and per-track tags can be written or updated during a remux, and statistics tags that describe bitrate and frame counts can be left alone or replaced. Attachments such as fonts for styled subtitles travel with the file, and segment fields including the title and date are all editable.
One companion program in MKVToolNix reports the internal structure of a Matroska file in as much detail as you need, which is how to find out whether a track is really what its name claims. Another extracts selected tracks, chapters, tags, attachments, and timestamp files back out as separate files.
The interface in MKVToolNix covers multiplexing, header editing, chapter editing, and file inspection, and it keeps a queue so several jobs can run one after another. Source files, track choices, and every option can be saved and restored, and command line arguments can be viewed for later reuse.
Browse historical releases and previous builds.
Common questions about MKVToolNix.
Combining or reshaping Matroska files without re-encoding: merging separate video, audio, and subtitle streams into one container, splitting a file, joining parts, correcting track labels and languages, and inspecting what a file actually contains. It edits containers rather than the picture.
No. The streams are copied byte for byte into the new container, so the audio and video data is unchanged and there is no generation loss. Only metadata, track flags, chapters, and the container structure are rewritten, which is why a remux of a large file finishes in seconds.
Yes, within limits. A delay can be applied to any track when the file is muxed, which handles a constant offset. If the drift grows over time, a stretch factor corrects it, and both adjustments apply to the subtitle timestamps without re-encoding anything.
mkvmerge builds a new file from one or more sources, which takes time proportional to the amount of data copied. mkvpropedit edits an existing file in place and only rewrites the header and index, so changing a track name or the file title completes almost instantly.
Yes. A WebM mode restricts the output to the codecs the format allows, which means VP8 or VP9 video with Opus or Vorbis audio. Any other streams are rejected or removed, and only the chapter and tag elements permitted by the specification are written, so the result stays compliant.
No. The graphical interface exposes multiplexing, header editing, chapter editing, and inspection with the same options, and it can show the equivalent command line when you want to script a job later. The command line tools remain useful for batch work across many files.
MKVToolNix is a toolkit for creating, altering, and inspecting Matroska and WebM files. It merges video, audio, and subtitle tracks from different sources without re-encoding, and edits existing files in place.
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