Crop a video to 1:1. Square is the cross-post ratio: it fills a feed acceptably everywhere, which makes it the safe single export when a clip has to run on several platforms.
Where square still wins
4:5 takes more vertical space in an Instagram or Facebook feed, and 9:16 owns Reels, Shorts, and TikTok. Square holds the middle: no platform renders it badly, and none crops it further in a feed.
It also suits anything that is composed centrally by nature. Product turntables, logo animations, and overhead cooking shots lose nothing to a square frame.
The crop
A 1920x1080 frame cropped to 1:1 keeps 1080x1080, which is 56% of the original width. A 1080x1920 vertical clip keeps 1080x1080 and loses 840 px of height.
Fit with bars is the alternative: the full 1920x1080 picture sits 1920x1080 across the middle of a 1920x1920 frame with 420 px of background above and below.
1:1 and 9:16 are not interchangeable
A square clip uploaded to Reels or TikTok is padded to 9:16 by the platform, in bars you did not choose. When the destination is a full-screen vertical surface, export 9:16 and control the padding here.