How do I rotate a video the right way up?
Drop the file, pick 90, 180 or 270 degrees, check it in the preview, export. The rotation happens in your browser on your device — the recording is never uploaded, and no watermark ends up in the picture. The result is an MP4 that appears the right way up on practically every device and in every program.
Why is my video lying on its side at all?
Because phones do not rotate the picture; they store it exactly as the sensor captured it and write the orientation into the file as a note. The MP4 container provides a 3 × 3 transformation matrix in the track header for this, defined in ISO/IEC 14496-12 ("ISO base media file format"). Players that read this matrix rotate on playback automatically; older programs and some editing tools ignore it — and then the clip tips over.
Does rotating cost quality?
If only the metadata note changes, nothing is recomputed and nothing is lost. If the rotation has to be baked into the frames, because the target ignores the note, the video goes through decoder and encoder once — with the same small quality loss as any other re-encode.
Why only right angles — and what about mirroring?
Because a wrongly held camera produces exactly those errors: 90 degrees left, 90 degrees right, or upside down. Free angles would crop the picture or leave black corners; that is a job for an editing program.
Mirroring is available too, horizontally and vertically — the usual reason is front-camera footage that was stored the wrong way round. It is not rotation, though: no container format provides a flag for mirroring, so it has to be baked into the frames. That means one round of re-encoding, while pure rotation stays lossless — and text in the picture becomes unreadable when mirrored. The preview shows both beforehand, in the order they are applied: rotate first, then mirror.
Honest limits
MP4, MOV, WebM and MKV go in, MP4 comes out. When a re-encode is needed it takes roughly as long as the video runs, and the tab has to stay open. The cap is 30 minutes per file. After exporting, check the result once in the program the clip is meant to run in — that is where you reliably see whether the rotation note is respected.