updated
Aug 24, 2026
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MKVToolNix is the free open-source toolkit for rebuilding MKV files, letting you merge, split, retag and extract tracks without re-encoding, because it rewrites only the container and leaves the video and audio streams untouched.
MKVToolNix rebuilds MKV files without ever re-encoding what is inside them.
Merge several clips into one file, drop in a subtitle track, strip out a commentary track you will never listen to, fix the chapter markers, or pull a single audio stream back out again.
Every one of those operations finishes in seconds and costs you nothing in quality.
Why It Never Costs You Quality
A video file has two separate parts: the compressed streams, and the container holding them. MKVToolNix only ever touches the second one.
Tools like HandBrake and Shutter Encoder decode your video and compress it again, which takes time and loses a little quality every pass.
MKVToolNix copies the streams across untouched and writes a new wrapper around them. The guide on what remuxing actually means covers the distinction properly, and it is worth ten minutes if the words keep tripping you up.
The practical upshot. Remuxing a two-hour Blu-ray rip takes about as long as copying the file, because that is essentially all it is doing.
What You Can Actually Do With It
| Task | What it solves |
|---|---|
| Merge | Combine video from one source with better audio from another, or fold external subtitle files into the video they belong to |
| Add and remove tracks | Add a second language, attach SRT or ASS subtitles, or delete the commentary track eating 400 MB |
| Set defaults and flags | Tag each track with its language and mark which one plays first, so your player stops picking the wrong audio |
| Split | Break an oversized recording by size, duration or chapter, for storage that will not take one huge file |
| Edit chapters and metadata | Add chapter markers and proper track names, which media libraries and players then display |
| Extract | Pull any single video, audio or subtitle stream back out as its own file |
The default-track flag is the one people discover late. If a film keeps starting in the wrong language, or forced subtitles never appear, the flags inside the file are usually why, and this is where you fix them.
Recent Changes
Version 101.0 was published on 24 August 2026. Its release notes have not gone up yet, so the most recent documented changes are still 100.0's, from 5 July.
Two of them matter more than the rest if you work with disc rips or AAC audio.
- AAC detection fix. mkvmerge was mis-detecting ADTS streams that did not start on a frame boundary, a regression that had been present since v87.
- Blu-ray playlist fix. When reading MPLS playlists, mkvmerge could skip content at the start of the main M2TS file - but only on discs carrying HDMV TextST subtitles.
- PowerShell actions on Windows. Job-queue actions can now run a PowerShell script, with the MTX_ variables exposed through the environment.
- Job queue search. You can now search jobs by description, output, warning or error text.
- MP4 track names. Names are now read from the track's user-data atoms when they are present.
- New scalable icons across all the applications, replacing the old bitmap set.
Three Tools in One Download
Most people only ever open the first of these.
- MKVToolNix GUI is the graphical front end, and it covers everything in the table above.
- mkvmerge is the command-line multiplexer underneath it, which is what you script against for batch work.
- mkvinfo reports the internal structure of a file, in more Matroska-specific detail than MediaInfo gives you.
Formats It Will Read
Input is not limited to MKV. It also reads MP4, AVI, MOV, MPEG, WebM, FLV, OGG and FLAC, which is what makes it useful for pulling scattered sources into one organised file.
On the video side it handles H.264, H.265/HEVC, VP8, VP9, AV1, MPEG-1/2/4, Theora, and the legacy DivX and Xvid material. Audio covers AAC, AC3, DTS, FLAC, MP3, Vorbis, Opus, TrueHD and PCM, and subtitles run to SRT, ASS/SSA, VobSub, PGS and WebVTT.
WORTH KNOWING
Remuxing does not make a file playable. If the codec inside was unsupported before, it stays unsupported after, because nothing about the stream has changed. Re-encoding is the only fix for that.
If a finished file will not play, the guide on why some MKV files refuse to play works through the usual causes, and the Codec Finder will name the codec a specific file needs.
Where It Sits Next to Everything Else
| Tool | Use it instead when |
|---|---|
| HandBrake or VidCoder | You need the file smaller, or in a codec it is not currently in |
| LosslessCut | You just want to trim one file quickly and never open a track list |
| MP4Box (GPAC) or MP4Forge | The container you are rebuilding is MP4 rather than MKV |
| FFmpeg | You want one command-line tool for remuxing and encoding both |
| Subtitle Edit | The subtitles themselves need retiming or correcting, not just attaching |
After a Disc Rip
This is where MKVToolNix earns its keep. MakeMKV gets the disc onto your drive intact, but it gives you everything on it, including audio tracks and subtitles you have no use for.
MKVToolNix is the second pass: strip what you do not want, attach better subtitles, fix the chapters, set the default tracks, then save. If MakeMKV itself is the problem, the piece on MakeMKV's site going down explains what the error actually means.
For playback afterwards, VLC Media Player handles MKV natively and needs nothing added to it.
For DirectShow players that do not, the K-Lite Codec Pack or a dedicated Matroska Splitter fills the gap.
Builds and Requirements
Windows builds come as an installer or a portable archive, in both 64-bit and 32-bit. Take the 64-bit installer unless you know otherwise, or the portable archive if you would rather not touch the registry.
The supported floor is Windows 10 or Windows Server 2016 and newer. On Linux, the AppImage runs without installation on any distribution shipping glibc 2.28 or later.
macOS needs 13 Ventura or newer, and has done since version 90. Those builds are produced by a community contributor rather than the project maintainer, which is why the Mac version sometimes trails the Windows one by a release.
Hardware demands are negligible everywhere. Nothing is being encoded, so a large file is bounded by how fast your disk can copy it rather than by your processor.
Quick questions
Is MKVToolNix free?
Yes, and open source. There is no paid tier, no watermark and no nag screen.
Does it reduce quality?
No. It copies the streams across bit for bit and only rewrites the container around them, so the output is identical to the input.
Can it convert MKV to MP4?
Not directly - it only writes Matroska files. For MP4 output use MP4Box or FFmpeg, both of which can remux losslessly the same way.
Why is my file still not playing after remuxing?
Because remuxing does not change the codec. If your player could not decode that video or audio before, rewrapping it changes nothing, and re-encoding with HandBrake is the real fix.
How do I stop films starting in the wrong language?
Open the file, set the language tag on each audio track, and mark the one you want as default. Save, and your player will follow it.
Can I use it without the interface?
Yes. mkvmerge is the command-line tool that does the actual work, and it takes the same options the GUI builds for you behind the scenes.
The wider video encoders category covers the re-encoding tools to reach for when a container rebuild is not enough.
Download MKVToolNix 101.0 and see how it handles your files, or tell us how you got on if you have used it before.
ME
the swiss army knife for Mkv files.
just awesome!
just awesome!
OC
Perfect. I love this site.
OP
Great tool!
I was unable to extract a subtitle. On the "Extract" says he is not yet functional
WHO
I was unable to extract a subtitle. On the "Extract" says he is not yet functional
WHO
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