Remux versus re-encode: converting a video without quality loss

Converting a video without any quality loss comes down to a single choice: remux or re-encode. A remux copies the existing video and audio streams into a new container untouched, so the picture is identical to the source and the job finishes in seconds. A re-encode decodes those streams and compresses them again, which is sometimes unavoidable but always costs a little quality. Knowing which one a given file actually needs is the whole game, and on a Mac you can do either.

What remux and re-encode actually mean

Every video file is really two things stacked together. The outer layer is a container: MP4, MOV, MKV, AVI, and the rest are containers, boxes that hold streams and label where each one starts. Inside sit the streams themselves, the compressed video and audio, each encoded with a codec such as H.264, HEVC, AAC, or AC-3. All of the quality lives in the streams. The container is just packaging.

A remux, short for re-multiplex, opens the box, lifts the streams out exactly as they are, and drops them into a different box. Not a single byte of the picture is decoded or recompressed, so quality cannot change. It is the video equivalent of moving a document into a new folder. Because nothing is processed, a full movie remuxes in seconds and the output is within a fraction of a percent of the original size.

A re-encode does the opposite. It decodes a stream back to raw frames and compresses it again with a codec. Compression like this is lossy: it throws away detail your eye is least likely to miss, and it does so every single time. Re-encode a file that was already re-encoded and you stack the loss, which is why repeated conversions look progressively softer. A re-encode is unavoidable when the target format cannot carry the codec you have, but it is never free.

How to tell which route your file can take

On a Mac the sensible target is MP4 or MOV, the two containers that QuickTime, Photos, Messages, and iPhones all play natively. Whether you can remux into one of them depends entirely on what is inside your file. Check the streams first, either in the codec information panel of a media player or with ffprobe in Terminal if you have ffmpeg installed.

Then match what you find against what MP4 and MOV accept:

• Video is H.264 or HEVC (H.265). Both containers hold these. Copy the stream unchanged. Lossless.
• Video is VP9 or AV1. These belong to web formats like WEBM, and Apple players will not reliably play them inside an MP4 even where the container technically allows it. The video has to be re-encoded to H.264 or HEVC.
• Audio is AAC, MP3, or AC-3. Copy unchanged.
• Audio is Opus, Vorbis, FLAC, or DTS. Not reliably supported by Apple players in MP4 or MOV. Re-encode just the audio to AAC and leave the video alone.
• Subtitles are SRT. Can be carried as MP4 text subtitles. Styled ASS or SSA subtitles cannot travel in MP4 at all, so keep them as a separate file.

Most everyday files, from screen recordings to camera clips to downloads, turn out to be H.264 or HEVC paired with AAC or AC-3. That combination is a pure remux, so the “converting a video without quality loss” case is the common one, not the rare one.

The lossless route: remux in Terminal

ffmpeg is the open-source engine that most converters are built on. It is not on a Mac by default, but a package manager installs it in one line, or you can download a build from the ffmpeg project. Once it is there, the remux is a single command.

1. Open Terminal.
2. Type ffmpeg -i and drag your video file onto the window so its path fills in.
3. Type -c copy and then a name for the output ending in .mp4 or .mov, for example ~/Desktop/clip.mp4.
4. Press Return. Even a long video finishes in a few seconds, because nothing is being decoded.

-c copy tells ffmpeg to copy every stream verbatim. If it stops with a complaint that a codec is not supported in the container, you have one of the incompatible streams above, and you adjust rather than give up: copy the video and re-encode only the audio with -c:v copy -c:a aac -b:a 192k, or drop subtitles the container cannot hold with -sn. The video stream stays untouched either way, which is the part that matters. To confirm the remux worked, compare file sizes: a true stream copy lands within a whisker of the source, and it opens in QuickTime.

Converting without the command line

If Terminal is not how you want to spend the afternoon, Manatee does the same conversion from a drag onto its menu bar icon, its window, or a right-click in Finder under Services. Choose Convert, pick MP4 or MOV as the target, and drop the file in, or a whole folder of them at once. Each job shows its progress, and the new file is written alongside the original, which is never modified.

Manatee bundles its own copy of ffmpeg and runs entirely on your Mac, so there is nothing to install and nothing is uploaded. It works out the codec decisions for you, so a file with DTS audio or an odd stream comes out as an MP4 or MOV that plays without you typing a command. Two honest notes. First, MP4, MOV, and GIF are the video targets it writes; it does not output MKV, AVI, or WEBM, so if you specifically need one of those containers, this is not the tool for that job. Second, if you want to verify a byte-for-byte stream copy yourself, the -c copy route above is the one that gives you that certainty on screen. For the everyday goal of a compatible file with no visible loss, either path gets you there.

One thing to avoid: do not re-encode a file just to change its container. If all you need is MP4 or MOV playback and the streams are already compatible, the conversion should be a copy, not a fresh compression. Shrinking a video to fit a size limit is a separate, deliberate re-encode, and it is worth doing from the best source you have rather than on top of a copy that was already squeezed.

Questions

Does a remux ever change the picture at all?

No. A remux copies the compressed video stream without decoding it, so the frames are bit-for-bit identical to the source. If the picture looks different, something was re-encoded, not remuxed.

My converted file came out much smaller than the original. What happened?

A large size drop is the signature of a re-encode: the stream was recompressed, usually at a lower bitrate. A remux keeps the size within a fraction of a percent. A much smaller file means quality was traded away somewhere, whether you asked for it or not.

Can I remux between MP4 and MOV without any loss?

Yes. MP4 and MOV are closely related containers that both hold H.264, HEVC, and AAC, so moving streams from one to the other is a clean copy. It is one of the safest conversions on a Mac.

Is there a way to re-encode without visible loss?

Close, but not truly lossless. A high-quality H.264 or HEVC re-encode at a conservative setting can look identical to most eyes, yet the bytes differ and a tiny amount of detail is gone. Do it once, from the best source, and never repeatedly.