Three ways to reframe
Blurred fill keeps the whole picture and fills the empty space with a soft, enlarged copy of the same video, the look most creators use when posting landscape clips to vertical feeds. Colour bars do the same with a plain colour of your choice. Crop to fill uses the whole frame and cuts off the sides or the top and bottom; drag the preview to keep the part that matters, such as a face or a product, in shot.
Sizes and limits for each app
TikTok, Reels and Shorts all use 1080 × 1920 pixels in 9:16. Square posts are 1080 × 1080, portrait posts 1080 × 1350 in 4:5, and YouTube videos 1920 × 1080 in 16:9. The tool checks the length against each app's current limit, such as 3 minutes for Shorts and Reels, and warns you when the source is too small to look sharp. Limits change from time to time, so if an upload is refused, check the app's help pages.
Saved as a standard MP4
Current Chrome, Edge and Safari save an MP4 with H.264 video and AAC sound, the format every social app accepts, using the browser's own video encoder. Browsers that can't make MP4 files save a WebM instead. Nothing is uploaded to do it, so drafts and unreleased clips stay private until you post them yourself.