MKVToolNix on a Mac: edit MKV files without re-encoding

MKVToolNix is a free, open-source software suite that creates, modifies, inspects, and demuxes Matroska (.mkv) video files on a Mac without re-encoding. Created and actively maintained by Moritz Bunkus, it is the universal standard utility for managing MKV media containers.

When you need to add an external subtitle track to a movie, remove an unneeded foreign audio dub to save disk space, or toggle a forced subtitle flag, MKVToolNix performs these tasks in seconds. Because it manipulates the outer container shell rather than the compressed video pixels, the process causes zero quality loss and consumes virtually no battery.

What is inside the MKVToolNix suite on macOS?

MKVToolNix is not a single tool; it is a unified collection of specialized command-line utilities anchored by a modern graphical interface (MKVToolNix GUI):

• mkvmerge: The core multiplexing engine. It combines separate video streams, audio tracks, subtitles, chapter files, and font attachments into a standardized Matroska container.
• mkvextract: The demuxing engine. It extracts individual tracks (such as an audio track or subtitle file) and embedded attachments directly out of an existing MKV file into standalone documents.
• mkvpropedit: The instantaneous property editor. It modifies track headers, language codes, track names, and default or forced flags directly inside an existing MKV file in milliseconds without rewriting the multi-gigabyte media payload.
• mkvinfo: The technical stream inspector. It parses and prints the low-level Extensible Binary Meta Language (EBML) structure of a file.

Why is container editing better than re-encoding in HandBrake?

When casual Mac users want to modify a video file, their default instinct is often to open transcoding software like HandBrake.

Transcoding decodes every single video frame into raw uncompressed pixels, recalculates compression matrices, and re-encodes the entire movie from scratch. On a standard two-hour 4K HDR movie, transcoding consumes hours of processing time, turns on your Mac’s cooling fans, drains your laptop battery, and introduces generational compression artifacts that soften fine image detail.

Animation

Remux or re-encode: two ways to turn an MKV into an MP4

Remuxseconds · no quality lost

MKVVideoAudioMP4

Re-encodeminutes to hours · some quality lost

MKVVideoEncoderMP4

MKVToolNix does not touch the pixels or sound waves. It operates strictly at the container level—a process known as multiplexing or remuxing.

Think of an MKV file as a physical shipping container holding boxes of video, audio, and subtitle streams. When you use MKVToolNix to add an SRT subtitle or remove a foreign commentary track, it opens the shipping container, inserts or removes the designated box, and closes the container. The entire process takes only three to ten seconds, governed purely by your Mac’s drive read-and-write speed.

To learn more about the structural differences between wrappers and codecs, read our breakdown of MKV vs MP4.

How do you install MKVToolNix on an Apple Silicon Mac?

MKVToolNix is fully native on Apple Silicon (M1, M2, M3, and M4 processors) and runs on modern versions of macOS, including macOS 26 Tahoe.

You can install MKVToolNix using two legitimate, verified methods:

Method 1: Using Homebrew (recommended for power users)

If you have the Homebrew package manager installed on your Mac, open Terminal (/Applications/Utilities/Terminal.app) and install the graphical desktop app:

brew install --cask mkvtoolnix

If you only want the command-line tools (mkvmerge, mkvextract, mkvpropedit) for Terminal scripting:

brew install mkvtoolnix

On Apple Silicon Macs, Homebrew installs the standalone binaries directly into /opt/homebrew/bin/.

Method 2: Downloading the official signed disk image

If you prefer a traditional Mac installer:

1. Navigate to the official download repository at mkvtoolnix.download/macos/.
2. Download the latest Universal macOS disk image (e.g. MKVToolNix-103.0.0.dmg).
3. Open the disk image and drag MKVToolNix into your /Applications folder.

Moritz Bunkus cryptographically signs and notarizes the official macOS disk images, ensuring that Apple Gatekeeper allows the app to open without security warnings.

How do you add subtitles to an MKV file without re-encoding?

Adding a downloaded .srt or .ass subtitle file directly into an MKV movie is the most common use case for MKVToolNix GUI.

Follow these step-by-step instructions:

1. Launch MKVToolNix from your Applications folder or Spotlight (⌘Space).
2. Select the Multiplexer tool from the left navigation sidebar.
3. Drag your source video file into the Source files box at the top.
4. Drag your subtitle file (.srt or .ass) into the same Source files box.
5. In the Tracks, chapters and tags box below, click on your newly added subtitle track to highlight it.
6. In the Properties panel on the right side:
   • Set the Language dropdown to your subtitle’s language (e.g. eng for English).
   • In the Track name field, enter a clean label (e.g. Full Dialogue or SDH).
   • If you want this subtitle to display automatically when opening the movie, set “Default track” flag to Yes.
7. In the Destination file field at the bottom, choose where to save the output file.
8. Click the Start multiplexing button at the bottom of the window.

In less than ten seconds, MKVToolNix creates a new MKV file containing your video, all original audio tracks, and the newly embedded subtitle. If you ever need to extract that subtitle back out to an external file, follow our tutorial on how to extract subtitles from an MKV on a Mac.

How do you remove unwanted audio tracks to save disk space?

High-definition movie rips frequently bundle audio tracks in six or seven languages, along with director commentaries. A lossless Dolby TrueHD or DTS-HD Master Audio track can consume 3 to 5 gigabytes of disk space per language.

If you only listen to the original audio mix, removing foreign dubs can shrink a 40 GB movie file down to 32 GB in seconds:

1. In MKVToolNix GUI, open the Multiplexer tab and add your MKV file.
2. In the Tracks, chapters and tags list, review the audio streams. Each stream displays its codec, channel count (e.g. 5.1 or stereo), and language code.
3. Uncheck the boxes next to the audio languages and commentary tracks you do not want to keep.
4. Verify that your preferred audio track has the “Default track” flag set to Yes.
5. Click Start multiplexing.

MKVToolNix writes a clean, slimmed-down MKV container that retains the untouched video stream while omitting the gigabytes of unwanted audio.

How do you edit track flags in milliseconds with mkvpropedit?

If all you want to do is change a track’s language label or set a forced subtitle flag, creating an entirely new multiplexed file is unnecessary. You can modify the existing file in place using mkvpropedit.

mkvpropedit modifies Matroska container headers directly on disk without touching or copying the underlying media streams:

Using the graphical Header Editor

1. In MKVToolNix GUI, click Header Editor in the left sidebar.
2. Open your .mkv file.
3. Expand the Tracks tree and select your subtitle stream.
4. Change “Forced display” flag to Yes.
5. Press ⌘S to save. The file updates in less than 0.1 seconds.

Using the command line

You can achieve the same instant header update in macOS Terminal. To set the forced subtitle flag on the first subtitle track of a movie:

mkvpropedit "Movie.mkv" --edit track:s1 --set flag-forced=1

To update an unlabelled audio track to English:

mkvpropedit "Movie.mkv" --edit track:a1 --set language=eng

To understand why media players rely on these container flags to display foreign-language dialogue automatically, explore our guide on what forced subtitles are.

How should you play your edited MKV files on macOS?

Once you have customized your MKV files with MKVToolNix, you need a media player that fully respects the Matroska specification.

Apple’s QuickTime Player cannot open MKV files or read Matroska track flags. Rather than spending time remuxing every file into an MP4 container, download MacLC from the download page.

MacLC is a free, open-source player built for Apple Silicon Macs running macOS 26 Tahoe or later. It plays MKV containers out of the box, switches between audio tracks, and renders embedded vector subtitles through libass while keeping subtitles at SDR white over HDR.