Rotate MP4
A sideways video usually isn't sideways in the file at all. Phones record in landscape and store a small rotation setting that tells players which way up to show it; when that setting is wrong, or the phone guessed wrong, the video plays on its side. Drop the video here, pick 90° right, 90° left or 180°, and Wave rewrites the setting in a new MP4 - in seconds, without touching a frame, and without uploading anything.
What actually changes
MP4 files carry a display matrix in each video track's header: a few numbers that tell the player to turn the picture before showing it. An iPhone held upright records landscape pixels with a 90° clockwise turn in that matrix. Rotating by changing the matrix is instant and lossless, and nearly every current player honours it: browsers, phones, the Windows and macOS players, VLC, and smart TVs from the last several years.
Wave adds your turn to whatever the file already had. If a portrait clip shows on its side, the original setting was wrong or missing; choose the direction that makes it upright. The result plays the same way everywhere the original did, and the picture data is untouched.
When the video is re-encoded anyway - because you also set a file size, a lower resolution, an exact trim, or Make it play everywhere - Wave turns the pixels themselves instead and writes a file with no rotation setting at all. That version plays upright even in the rare player or editing tool that ignores the setting.
With a current version of ffmpeg, `ffmpeg -display_rotation -90 -i input.mp4 -c copy output.mp4` sets a 90° clockwise turn without re-encoding (the value counts anticlockwise, so -90 means clockwise; use 90 for anticlockwise and 180 for upside down). Older guides use `-metadata:s:v:0 rotate=90`, which recent ffmpeg versions ignore. To turn the pixels instead, re-encode with `-vf transpose=1` for clockwise or `-vf transpose=2` for anticlockwise.
Three steps.
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Step 1
Drop the video onto Wave above. On this page, the Options panel opens on Rotate.
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Step 2
Choose 90° right, 90° left or 180°.
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Step 3
Select Convert to MP4 and save the result. On its own, rotating takes seconds.
Answers, specifically.
Does rotating reduce quality?
No. On its own, rotating only changes the file's rotation setting; the frames are copied byte for byte. If you combine it with a size target or another option that re-encodes, the turn is applied to the pixels during that re-encode.
Which direction do I need?
If the top of the picture points to the left, choose 90° right; if it points to the right, choose 90° left; if it's upside down, 180°. The plan confirms the turn before you convert.
Why does my video play upright on my phone but sideways on my computer?
Some older players and video editors ignore the rotation setting and show the stored landscape pixels. A copy with the turn built into the pixels fixes that: in Wave, anything that re-encodes the video (a file size, a lower resolution, or Make it play everywhere for HEVC clips) writes the picture upright, so it plays the right way in every player.
Can I rotate on an iPhone?
Yes. In Photos, open the video, tap Edit, tap the crop icon, then the rotate button at the top left; each tap turns it 90° anticlockwise. Tap Done to save.
Can I rotate on a Mac without extra apps?
Yes. QuickTime Player has Edit > Rotate Left and Rotate Right. Save the file afterwards to keep the change.
Can I flip a video like a mirror?
Not in Wave. A mirror image is a different transform from a rotation; ffmpeg does it with `-vf hflip` (left-right) or `-vf vflip` (top-bottom), which re-encodes the video.