Muxing, short for multiplexing, is the process of combining separate video, audio, subtitle, and metadata streams into a single file. When you have a video track, an audio track, and maybe a subtitle file sitting as independent pieces, muxing wraps them together into one playable container like an MP4 or MKV. It doesn’t alter the quality of any stream. It just packages them.
How Muxing Actually Works
A video file isn’t one continuous ribbon of data. It’s built from small packets of video, audio, and other streams woven together in an alternating pattern, a technique called interleaving. A muxer takes your separate streams and slices them into these packets, then arranges them so a player can read a chunk of video, then a chunk of audio, then more video, in rapid sequence.
To keep everything in sync, each packet gets timestamps embedded in its header. There are two types: a decoding timestamp that tells the player when to decode the data, and a presentation timestamp that tells it when to actually display the frame or play the sound. These timestamps are what prevent audio from drifting out of sync with video. Without them, a player would have no way to line up lip movements with dialogue or match sound effects to on-screen action.
Muxing vs. Encoding
This is the distinction most people are looking for. Encoding compresses raw video or audio data, converting it from one format to another (or shrinking it). It’s computationally heavy and changes the actual media data, often reducing quality slightly in exchange for a smaller file. Muxing does none of that. It takes already-encoded streams and places them into a container without touching the underlying data. That’s why muxing is fast, often finishing in seconds, while encoding the same file could take minutes or hours.
A related term you’ll see is “remuxing,” which means taking streams out of one container and placing them into a different one. For example, extracting the video and audio from an MP4 and putting them into an MKV file. Again, no re-encoding happens, so there’s zero quality loss.
What Demuxing Is
Demuxing is the reverse process: extracting individual streams from a single container file. Every media player does this automatically during playback. It reads the container, separates the video, audio, and subtitle streams, then sends each one to the appropriate decoder. You can also demux files manually when you want to pull out just the audio track from a video, swap in a different subtitle file, or isolate a specific stream for editing.
Container Formats and What They Support
The container format you mux into determines which types of streams your file can hold. The three most common containers each have different strengths.
- MKV (Matroska): The most flexible option. It supports virtually every video codec (H.264, H.265, AV1, VP9, and many others), a wide range of audio formats (AAC, MP3, FLAC, DTS, TrueHD, Opus), and extensive subtitle support including SubRip, ASS, SSA, WebVTT, and PGS. If you need to combine multiple audio tracks or subtitle languages into one file, MKV is typically the best choice.
- MP4 (MPEG-4 Part 14): The most universally compatible format. It handles the major video codecs (H.264, H.265, AV1) and common audio formats (AAC, MP3, AC-3, Opus, ALAC), but its subtitle support is more limited, generally restricted to timed text formats. MP4 plays natively on nearly every device, browser, and streaming platform.
- AVI: An older format with narrower codec support. It lacks newer video codecs like H.265 and has limited subtitle handling. AVI still works, but there’s rarely a reason to choose it over MP4 or MKV for new projects.
One thing that surprises people: the container itself adds almost no size to your file. Container overhead is less than 0.1% of total file size for both MP4 and MKV. If you mux the same video and audio streams into either format, the resulting files will be nearly identical in size. The difference between a 2 GB and a 4 GB video file comes down to encoding settings, not the container.
Common Muxing Tools
Two tools dominate the space. MKVToolNix is a free, dedicated application for creating and editing MKV files. It has a graphical interface where you drag and drop your video, audio, and subtitle files, configure track options like language tags and default tracks, then hit “start muxing.” It’s the go-to for anyone working with MKV containers, and the current version (mkvmerge 88) is actively maintained.
FFmpeg is a command-line tool that handles nearly every media task imaginable, muxing included. It supports a broader range of container formats than MKVToolNix and can output to MP4, MKV, AVI, and dozens of others. The trade-off is that it has no graphical interface, so you need to type commands. A basic FFmpeg mux command copies streams from input files into a new container without re-encoding, finishing almost instantly.
Both tools produce identical quality output because neither one alters the underlying streams. The choice comes down to preference: MKVToolNix for a visual workflow focused on MKV files, FFmpeg for flexibility and automation across any format. Many people use both, downloading as MP4 and then remuxing into MKV with MKVToolNix to add subtitle files or additional audio tracks.
When You’d Mux a File Yourself
The most common scenario is combining separately downloaded or created streams. You might have a video file and a matching subtitle file you want bundled together so you don’t need to load the subtitles manually every time. Or you have a video with English audio and want to add a second audio track in another language. Muxing handles all of this without re-encoding, so the process is fast and lossless.
Remuxing is also useful when you need to change containers for compatibility. Some devices won’t play MKV files but handle MP4 fine. If the codecs inside are compatible (H.264 video with AAC audio, for instance), you can remux from MKV to MP4 in seconds. If the codecs aren’t compatible with the target container, you’d need to re-encode, which is a different and much slower process.