Short answer: design at 1640 × 624 px (twice the 820 × 312 desktop display size) and keep text inside the centre.
Why two sizes?
Facebook shows the cover at about 820 × 312 on computers and about 640 × 360 on phones. On phones the cover is taller and narrower, so the left and right sides are cut off. Designing at double resolution keeps text sharp on retina screens.
Other Facebook covers
| Type | Recommended size |
|---|---|
| Page / profile cover | 1640 × 624 |
| Group cover | 1640 × 856 |
| Event cover | 1920 × 1005 |
Tips
- Keep the lower-left area free on desktop — the profile picture sits close to it.
- Upload PNG for covers with text and logos; Facebook compresses JPGs harder.