Cropping is a compositional decision
Cropping removes pixels permanently, so the useful question is not how to crop but what to remove. The conventional guides exist because they work: placing the subject a third of the way across the frame rather than dead centre generally produces a more dynamic image, and leaving space in the direction a subject faces or moves reads as natural while cropping tight to that side feels constrained.
Horizons are the most commonly mishandled element. A horizon through the exact centre divides the image into two competing halves; placing it on the upper or lower third commits the picture to being about the land or about the sky. A horizon that is not level is the single most noticeable flaw in an otherwise good photograph, and is worth fixing before cropping rather than after, since rotation costs edge pixels.
For portraits, avoid cutting at joints — wrists, elbows, knees, ankles. A crop through a joint reads as an amputation, while a crop through the middle of a limb segment reads as a frame edge. This is a convention rather than a rule, but it is a strong one.
Aspect ratios that platforms actually use
Cropping to a platform's ratio yourself is almost always better than letting the platform do it, because automatic cropping picks the centre and the centre is frequently not the subject. Common targets: 1:1 for square posts and most profile pictures, 4:5 for the tallest ratio Instagram accepts in feed, 9:16 for full-screen vertical video and stories, and 16:9 for video thumbnails and most presentation slides.
Profile pictures deserve particular attention because they are usually displayed as circles. The corners of a square upload are cut off entirely, so anything important must sit within the inscribed circle — roughly the central 70 percent. A logo cropped to a square with tight margins loses its corners once the platform rounds it.
Open Graph images for link previews are conventionally 1.91:1, commonly produced at 1200 × 630. The social card preview shows how a given image will actually appear once each platform crops it, which is faster than discovering the problem after posting.
Resolution after cropping, and metadata
Cropping reduces pixel count, and the reduction is easy to underestimate because it scales with area. Taking the central quarter of the frame linearly — half the width and half the height — leaves 25 percent of the pixels. A 12-megapixel photograph cropped that hard is a 3-megapixel one, which is still ample for screen use and marginal for a large print.
Check the output dimensions rather than trusting the on-screen preview, especially when cropping for print, where 300 ppi at final size is the target. A crop that looks fine on a monitor may be well below what an A4 print needs.
Cropping does not necessarily remove metadata. EXIF fields including GPS coordinates and camera serial numbers usually survive, so cropping a person or a document out of a photograph does not remove the record of where it was taken. Run it through the EXIF remover if the image is going anywhere public.