Rotate a video

MP4 and MOV both carry a transformation matrix beside the picture, and every player reads it before it draws a frame. Rotating a video properly means changing that number. The pixels are copied through untouched, which is why this takes milliseconds on a file that would take minutes to re-encode, and why the result is bit-for-bit as good as the original.

That is also why the file that comes out is the same size as the one that went in. Nothing was compressed again, so there is nothing to shrink and nothing to lose.

A phone films in one orientation and records the angle it was held at. Software that reads the flag shows the video upright; software that ignores it shows the raw frames, which is the sideways version. That is why the same file looks correct in the phone gallery and wrong in a video editor or on a website.

So a video is rarely "filmed wrong". It is usually filmed with a flag that something downstream did not read — and setting the flag explicitly, which is what this does, is what makes it agree everywhere.

A few places still ignore the flag: some older upload processors, some social platforms that re-encode on receipt, and some embedded players. If a video looks right here and wrong after uploading it somewhere, that is what happened.

The fix in that case is a re-encode with the rotation applied to the pixels — slower, and one generation of quality. It is not offered here, because doing it by default would make everyone pay for a problem most people do not have.

A file that carries a rotation flag and gets another quarter turn ends up with the sum of the two, not the second one. Turn a video that was already flagged 90 degrees by another 90 and it plays upside down, which is exactly the state people describe as “it rotated the wrong way”.

The flag is read before the new one is written, so what you choose here is applied on top of what the camera recorded. If a video looks upright in your player, it is already flagged and the quarter turn you want is probably not the one the raw frames suggest.

Windows Explorer, macOS Finder and most photo apps keep a cached thumbnail keyed on the file rather than on its contents. Rotate a video and the picture in the folder can stay stubbornly sideways while the video itself plays upright.

Opening the file settles the question, and it is worth doing before concluding the rotation failed. The caches rebuild on their own schedule; on Windows, emptying the thumbnail cache forces it, and on macOS the preview usually corrects itself once the file has been opened once.

The rotation matrix is a feature of the MP4 family, which includes MOV and M4V, because all three descend from the same ISO base media format. It is where phones record and it is what this page works on.

AVI has no equivalent field at all, and Matroska stores the idea differently and less consistently across players. A video in one of those has to be re-encoded to be turned, which is a slower operation costing one generation of quality — so if the file you are holding is an AVI, the instant lossless route this page describes is not available to it, whatever tool you use.