Facebook cover photo size

Facebook displays a cover at 851 × 315 on desktop and 640 × 360 on mobile — different aspect ratios, so the same file cannot fill both. Upload at 1702 × 630 for sharpness on high-density screens.

The two display sizes are the whole problem: desktop is wider and shorter, mobile is narrower and taller, and Facebook crops the same upload differently for each. What survives both is a centred region roughly 640 wide within the 851 — so text near the left or right edge is visible on a laptop and gone on a phone, where most of your audience is. On a personal profile the profile picture also punches into the lower left of the cover, which removes another corner. Upload at double resolution: Facebook re-compresses regardless, and giving it more pixels to start from produces a visibly cleaner result.