MKVToolNix
A Matroska toolkit for muxing video, audio, subtitles, chapters, tags and attachments into MKV files, inspecting tracks and changing container properties without re-encoding media.
What MKVToolNix does
MKVToolNix combines existing video, audio, subtitle, chapter, tag and attachment tracks into a Matroska container. It can inspect, extract and edit track properties without re-encoding the media. It is a container tool rather than a general transcoder or a way to make an unsupported codec playable on a target device.
Tracks, timestamps and playback
Output behavior depends on source timestamps, subtitle encoding, language and default flags, plus the decoder support of the target player. A container can hold a track that a television, browser or phone still cannot decode. Keep original media, use a separate output directory and test the beginning, middle and end on at least two target players.
Maintenance note
This page reviews MKVToolNix 100.0, Windows, Linux and macOS packages, track muxing, chapters, attachments, property editing and GPL-2.0-or-later boundaries. Content review date: 2026-08-23.
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MKVToolNix 100.0 lossless audio, subtitle and chapter muxing guide
Keep the original media untouched, mux one video, two audio tracks and a subtitle into a new MKV, set language and default flags, then verify sync, subtitles and chapters.
Before you start
- Prepare media you are allowed to use and record duration, frame rate, sample rate and start times for each input.
- Reserve at least one output-file size of extra space and keep the originals in an independent backup.
- Check the codecs supported by the target player because muxing does not change codec compatibility.
Installation steps
- 01
Install the 100.0 package
Choose the Windows 64-bit installer or portable archive, Linux package or AppImage, or the macOS universal image, and confirm architecture.
- 02
Create a separate output directory
Keep source files read-only or backed up, write into a different directory and use simple filenames during the first test.
- 03
Check Multiplexer settings
Confirm the MKV extension, temporary-space availability and safe completion action, and avoid any option that deletes source media automatically.
Quick start
- 01
Add sources and tracks
Add the main video and external audio or subtitle sources, then inspect type, codec, duration and track ID before disabling unneeded tracks.
- 02
Set language and flags
Assign language, name, default and forced flags for each track, keeping one sensible default per type and using forced subtitles only where needed.
- 03
Mux and verify fully
Review the job log and exit status, then test sync, subtitle encoding, chapters and track switching at the beginning, middle and end in target players.
Usage tips
- Lossless muxing means no media re-encode; it does not guarantee correct timestamps, flags, subtitle encoding or player compatibility.
- Header editing can be fast but still risks a damaged file after interruption or a wrong target; make a backup before changing properties.
- Fonts, covers, subtitles and audio tracks can have separate redistribution rights from the software license.
Troubleshooting and uninstall
Why are audio and video out of sync after muxing?
Compare start times, duration, frame rate and delay on each source, then test a short sample. Linear drift usually needs timestamp or resampling work rather than one fixed delay.
Why is a track silent or missing in a player?
Inspect the actual codec and container flags, then check decoder support on the target device. A valid MKV track is not proof that every player can decode it.
- Save jobs and verification notesKeep reusable job settings, chapter and subtitle sources, and player-test results until the output is confirmed.
- Remove the program and temporary jobsFinish the queue, uninstall or remove the portable directory, and clean failed temporary outputs while retaining originals and approved deliverables.
Frequently asked questions
Does MKVToolNix reduce video quality?
Normal muxing does not re-encode audio or video, so quality usually remains unchanged. Output size can still vary with track selection, attachments and container overhead.
Can it convert MKV to MP4?
Its main job is creating and editing Matroska files, not general transcoding. Use a tool with the target container and codec support for MP4 conversion.
Why is a subtitle track not shown?
Check that the track is enabled and that language, default or forced flags, encoding and player support match the intended behavior.