What each ratio buys you
9:16 fills the entire phone display. It is the native shape of TikTok, Reels, Shorts, Stories and Spotlight, and nothing else competes for the viewer's eye while it plays.
4:5 is the tallest shape a standard feed post is allowed. It takes roughly 25% more vertical space than a square without triggering the full-screen player, which makes it the strongest choice for an in-feed image or video post.
1:1 is the safe universal. It never gets cropped awkwardly by orientation and works identically as a feed post and a grid tile, but it claims the least screen of the three.
The cropping problem nobody plans for
A 9:16 video posted where a square is expected does not get letterboxed - it gets centre-cropped, and whatever was near the top or bottom disappears. That is usually your caption or your face.
The reverse is worse: a 16:9 clip forced into a vertical feed leaves two black bands and looks like reposted content, which several platforms actively deprioritise.
Shoot once, publish everywhere
Frame the shot with a 1:1 safe area in the centre even when you are recording 9:16. Anything essential - face, product, on-screen text - stays inside that centre square.
Then crop outward for each destination rather than trying to rescue a badly framed original. A centre-safe 9:16 master crops cleanly to 4:5 and 1:1; a tightly framed one does not crop at all.
Keep the highest-resolution master you have. Cropping discards pixels, so cropping from 1080×1920 to 1:1 leaves you 1080×1080 - exactly enough, with nothing spare.
The short version
Default to 9:16 for short-form feeds, 4:5 for standard feed posts, 1:1 only when one asset has to work everywhere. Frame centre-safe and crop down rather than shooting separately for each.