About this tool
Drop an MP4, MOV, WebM or MKV file, press "Rotate left", "Rotate right" or "180°" until the preview looks right, and export. The whole thing takes a few seconds even for a long video, because nothing is decoded: the picture and sound inside the file are copied exactly as they are, and only the orientation flag in the container is rewritten. The file never leaves your computer. This is the fix for the phone video that plays sideways on a computer or TV, or the clip you shot holding the phone upside down.
What the orientation flag is
A phone does not turn the picture when you hold it sideways. The sensor records the frames the way it sees them, and the file carries a small note — "display this turned 90°" — that every player reads before drawing a frame. That is why a clip can look fine on the phone and sideways somewhere else: one player read the note, the other did not. This tool reads the note your file already has, adds the turn you chose, and writes the result back. Choosing 90° on a clip that already says 90° gives 180°, so the preview always shows what the final file will say. After exporting, the page reads the new file again and refuses to hand it over if the flag does not match what you asked for.
Why WebM and MKV come out as MP4
MP4 and MOV have an orientation field that players have agreed on for years, so those files stay in their own container. WebM and MKV have no such field that players consistently honour, so a rotated WebM or MKV is written as an MP4 instead — the packets are still untouched, only the box around them changes. For now this only works when the video is H.264, HEVC, VP9 or AV1 and the audio is AAC, Opus, MP3 or FLAC. VP8 video and Vorbis or PCM audio can technically be put in an MP4, but many players do not expect them there, so the tool stops and names the codec instead of producing a file that might not play.
Honest limits
Because only the flag changes, the result depends on the player honouring it. Most players and phones do; some players, older TVs and some website upload pipelines may ignore it and still show the video sideways — we have not tested which. Actually rotating the pixels would need re-encoding, and this page never re-encodes. Rotation is limited to multiples of 90°: no free angles, no mirror or flip. One video per run. If the preview stays black, your browser cannot decode that codec (often HEVC on a computer without an HEVC decoder) — the rotation still works and the result plays wherever the original did.
On desktop Chromium the output streams straight into a file you choose, so a large video never sits in memory; other browsers build it in memory with a limit of about 1 GB. The output is a fresh container that carries only the video and audio packets, not the source's descriptive tags such as title, date or GPS location. If you also want to cut the clip, Video Trimmer does the same copy with an in and out point; Mute Video drops the sound; Video Info shows what a file says about itself, including its current orientation.