Shooting one clip that works in three sizes
The same clip, headed for Reels, the feed, and YouTube: three different shapes. Reels and Shorts run tall (9:16), the feed runs square (1:1), YouTube runs wide (16:9). Post landscape footage into a vertical slot and there are black bars top and bottom. Go the other way, vertical footage into a landscape slot, and the gaps show up on the sides instead.
Making all three versions is the easy part, and it comes later. What matters first is what gets decided at shoot time: cropping can happen afterward, but nothing fills in footage that was never framed to begin with.
Shoot for the narrowest frame
Of the three, vertical (9:16) is the narrowest. Landscape footage can be cropped down to vertical, but vertical footage can't be stretched out into landscape without looking wrong. So the shot gets framed so everything that matters fits inside the vertical frame, and cropping it down to square or landscape afterward becomes a matter of picking a slice, not scrambling for footage that was never captured.
Keep the subject centered
A person or object sitting at the edge of the frame gets cut off entirely once it's cropped to square or vertical. Centered, it survives no matter which shape it lands in.
Leave the top and bottom clear
There's caption space to think about, plus, on Reels and Shorts, the account name and like button sitting near the bottom of the screen. A key moment pressed right up against the very top or bottom of the frame gets covered by one of those. Leaving room up top and at the bottom while filming means captions can go in later without reframing the shot.
Make it work with the sound off
A lot of people watch feed and Reels content muted. Captions help, but it's better if the visuals alone carry the story. A scene that only makes sense because of something said out loud disappears entirely with the sound off.
Why the shapes split this way
Vertical (9:16) comes from Reels and Shorts, watched with the phone held upright. Square (1:1) is the shape that settled in on the feed, where saving vertical space mattered. Landscape (16:9) traces back to YouTube and TV, where the screen itself was always wide. Each platform ended up with a different shape because people hold their phones, or sit in front of their screens, differently.
Need the exact width and height for a given ratio? The aspect ratio tool works it out: enter the width, and the height follows.