The Facebook Cover Photo size (851×315px)
The Facebook cover photo sits at the top of profiles and Pages at 851x315 pixels, a wide 2.7:1 banner. Uploading at exactly this size stops Facebook from stretching or auto-cropping your artwork, which is the number one cause of covers that look off-center or unexpectedly zoomed.
The tricky part is that the cover renders differently across devices. On desktop you see the full 851x315 rectangle, but on mobile Facebook crops in and shows a taller, narrower slice of the center. That means the left and right edges visible on desktop are hidden on phones, where a large share of your audience is.
On personal profiles the circular profile picture also overlaps the bottom-left corner of the cover, obscuring whatever sits behind it. Design with a safe central band that survives the mobile crop, and keep essential text and logos out of the bottom-left where the avatar lands. Treat the outer edges as desktop-only decoration.