How it works
Instagram accepts 1:1, 4:5 and 1.91:1, but the square is the only one that never gets re-cropped: the profile grid shows every post as 1:1 regardless of how it was uploaded.
1080 px is the widest image Instagram actually serves. Uploading a 4000 px file does not improve anything — it just gets downscaled and recompressed on their side, usually with more visible artefacts than if you had resized it yourself.
Frequently asked questions
Should I post 1:1 or 4:5?+
4:5 takes more room in the feed, so it usually gets more attention. But the profile grid crops it back to a square, so anything near the top or bottom edge disappears there. Square if the grid matters to you, 4:5 if the feed does.
Why does my photo look worse after uploading?+
Instagram recompresses everything. If you upload larger than 1080 px it also downscales first, and the two steps together are what you see. Resize to 1080 yourself and you skip one of them.
Does a bigger file mean better quality?+
Not once it is on Instagram. Beyond 1080 px wide the extra data is discarded on their side — a 3 MB upload and a 400 KB one end up as the same served image.