MKVToolNix
FreePack subtitles and multiple audio tracks into one file, no re-encoding
A toolset for the MKV container format. Merge video, multiple audio tracks, multiple subtitles, cover art and chapters into one mkv file, or split them back out; remove unwanted tracks, change track language and default flags, and reorder them. Nothing is re-encoded, so it runs at close to file-copy speed.
Why we picked it
One common confusion is worth clearing up first: containers and codecs are different things. mp4 and mkv are boxes; H.264 and AAC are what is inside. Changing the box does not touch the contents, which is why it is lossless and nearly instant — only re-encoding costs quality and time.
MKVToolNix changes boxes and organizes what is inside them.
The most common uses:
- Embed external subtitles. Combine an mp4 and an srt into a single mkv, so copying it to a TV, tablet or USB stick moves one file and the subtitles never get lost. Note that these are soft subtitles — the player can toggle them and change fonts — unlike burning them into the picture with FFmpeg.
- Merge multi-language audio. A dub and the original soundtrack in one file, switchable during playback, and likewise several subtitle tracks.
- Strip unwanted tracks. A download carrying seven audio tracks and ten subtitles you will never use can be halved in size by removing them — losslessly.
- Fix track metadata. Wrong default language, subtitles not enabled by default, garbled track names — all editable as flags without re-encoding anything.
- Split and join files by size, duration or chapter, or reassemble segmented files into one.
The MKVToolNix GUI is drag-and-drop: pull the video, audio and subtitle files in together, tick
which tracks to keep, set languages and default flags, press start, and it finishes in seconds.
A command-line version, mkvmerge, handles batches.
To be clear about:
- Output is MKV only. It is a Matroska tool and cannot write mp4. If you need mp4 — some older TVs and phones accept nothing else — remux with FFmpeg.
- Think about compatibility. MKV is fine on computers and mainstream players, but some older TVs, car systems and iOS native players handle it poorly. Consider the recipient's device before sending.
- It does not transcode. Changing resolution, bitrate or codec is HandBrake's or FFmpeg's job.
- Source is hosted on Codeberg (formerly GitHub); download from the official mkvtoolnix.download.
Like LosslessCut, FFmpeg and HandBrake, it is the kind of tool you forget about until the moment nothing else will do.