Why your video plays sideways in the first place
Phones record the sensor data the same way regardless of how you're holding them, then write a rotation flag into the file metadata describing which way is up. Most modern players read that flag and rotate on the fly — but plenty of them don't. PowerPoint, older Windows Media Player, some Slack previews, corporate VPN-routed video tools, half the embedded players on niche CMSes. The fix is to bake the rotation into the actual pixels so every player gets it right. That's what this tool does.
Rotate vs flip — the difference matters
Rotate turns the video around the center, like spinning a photo on a desk — text and faces stay readable, just oriented differently. Flip horizontal mirrors left-to-right — text becomes unreadable backwards, but it's the right fix for selfie-camera footage that recorded you mirrored. Flip vertical mirrors top-to-bottom and is mostly useful for action-cam footage from upside-down mounts (where 180° rotation is usually what you actually want instead, but flip-vertical occasionally has its day).
After rotating, double-check the aspect ratio
Rotating 90° swaps width and height — a 1920×1080 source becomes a 1080×1920 output. That's probably what you want (you wanted a vertical video, not a sideways one), but if your destination needs a specific aspect ratio, run the result through the resize tool afterwards. Flipping doesn't change dimensions at all, so flip-then-upload is usually the whole pipeline.
Audio is preserved exactly
We re-encode only the video stream (CRF 22 H.264, visually transparent). Audio is copied byte-for-byte with -c:a copy — no quality loss, no re-encode artifacts. If your source had a music track or a precisely-mixed voiceover, it arrives at the output identical. Recording fresh? Use Clipy and pick orientation at capture so you never have to come back here.