Instagram post size: 1080 by 1080

The square is the format the platform was built around, and it persists for a structural reason rather than a nostalgic one: the profile grid is a grid of squares. A portrait post is centre cropped to a square for that grid, so anything important near the top or bottom of a tall image is missing from the view a new visitor sees first.

At 1080 by 1080 the frame is 1.17 megapixels, noticeably less than the 1.46 of the portrait frame. Since the feed gives both the same width, the square simply occupies less vertical space on the screen, which is a real trade: less attention per post, but a composition that survives the grid crop untouched.

The square is also the most forgiving canvas for reuse. It sits inside a portrait frame with padding, it crops out of a landscape frame from the centre, and it works as an avatar. A design system that starts square and expands outwards tends to survive a platform changing its mind; one that starts at an unusual ratio does not.

Both the image resizer and the social post generator carry a 1080 by 1080 preset, and the cropper has a 1:1 ratio for squaring a source first. That is the shortest path from an arbitrary photograph to this frame, and cropping before resizing is what avoids the horizontal squash you get when a rectangular source is scaled straight into a square.