Remuxing means copying the existing video, audio and subtitle streams into a new container without re-encoding them. The picture and sound stay exactly the same; only the wrapper changes. MOV to MP4 and MKV to MP4 are common remuxes when the destination container supports the codecs already inside.
A simple test: if the operation finishes much faster than the video's running time and the codec has not changed, it was probably a remux.
Remuxing vs. transcoding
| Remuxing | Transcoding | |
|---|---|---|
| What changes | Container, tracks or metadata | Codec, bitrate, resolution or frame rate |
| Re-encodes video | No | Yes |
| Quality loss | None | Usually some |
| Speed | Usually limited by disk or network speed | Limited by encoder speed |
| File-size change | Usually negligible | Can be substantial |
| Example | MKV to MP4 with H.264 copied | H.265 to H.264 |
The distinction matters because a tool can call both operations “convert.” If
it copies H.264 video from MOV into MP4, it is remuxing. If it decodes that
video and encodes it again, it is transcoding — even when the filename still
ends in .mp4.
Does remuxing reduce file size?
Barely. Container overhead is small, so changing the wrapper usually changes the total by only a fraction of a percent to a couple of percent. Removing an unwanted audio or subtitle track can save more, but the video stream itself is still the same size.
We checked this with a 10-second 1080p H.264/AAC reference file on 13 October 2026. Remuxing MP4 to MKV with FFmpeg stream copy changed the file from 5,344,289 bytes to 5,338,400 bytes — 0.11% smaller — and took 0.58 seconds including FFmpeg startup. The copied video and audio stream MD5 hashes matched the originals exactly. One short sample does not establish a universal container-overhead ratio; it demonstrates the important part: the media bytes did not change.
If you need a meaningfully smaller video, use a video compressor. That requires re-encoding; remuxing is the wrong operation for the job.
How to remux with FFmpeg
To copy every compatible stream from MKV into MP4:
ffmpeg -i input.mkv -map 0 -c copy output.mp4
-c copy selects stream copy instead of an encoder. -map 0 asks FFmpeg to
include every stream from the input. MP4 cannot hold every codec or subtitle
format that MKV can, so this command can fail when the destination container
does not support one of those streams.
To keep the video and only the first audio track:
ffmpeg -i input.mkv -map 0:v:0 -map 0:a:0? -c copy output.mp4
The ? makes the audio mapping optional. This is useful for dropping
commentary tracks or extra languages without touching picture quality.
For an MP4 intended for web playback, move its metadata to the beginning:
ffmpeg -i input.mp4 -map 0 -c copy -movflags +faststart output.mp4
That enables playback to start before the whole file downloads. It does not compress or re-encode the video.
When remuxing is the right answer
- A compatibility problem, not a size problem. A player rejects an MKV even though it supports the H.264 and AAC streams inside. Remuxing to MP4 can fix that without quality loss.
- Removing tracks. Drop a commentary track, an unwanted subtitle stream or audio entirely.
- Fixing container metadata. Rewrite timestamps, rotation flags, indexes or a misplaced MP4 metadata block.
- Changing delivery format. Package an already-compatible stream in the container required by a player, browser or editing application.
When remuxing will not help
- You need a smaller video. The encoded video stream accounts for nearly all the bytes. Re-encode it instead.
- You need another codec, resolution, bitrate or frame rate. Every one of those changes requires transcoding.
- The destination container cannot hold the existing streams. Putting an unsupported codec inside MP4 can fail outright or produce a technically valid file that the target device still cannot play.
- The source is damaged at the codec level. Rewriting the wrapper cannot repair corrupt encoded frames.
The common confusion is worth stating plainly: “convert MOV to MP4” is often a remux, completes in seconds, and correctly changes almost nothing about file size. That is not the tool failing. That is remuxing working.
FAQ
Is remuxing lossless?
Yes. A true remux copies the encoded media streams instead of decoding and re-encoding them. Container metadata changes, but video and audio quality do not.
Does remuxing change the codec?
No. H.264 remains H.264, AAC remains AAC, and so on. A codec change is transcoding, not remuxing.
Is converting MKV to MP4 always a remux?
No. It can be a remux when MP4 supports every stream you choose to keep. If the source contains an incompatible video, audio or subtitle codec, the tool must transcode that stream, drop it or fail.
Why did MOV-to-MP4 finish instantly but stay the same size?
Because the tool probably copied the existing streams into a new container. That is the expected remuxing result: fast, lossless and almost the same size.