For those of you designing cover images for Facebook, I have created this image in order to aid your design. You can open it in Photoshop (or GIMP or whatever your design program of choice is), add it as a layer, and then add your content over it. Keep in mind that this template is for Personal Facebook pages only, business pages will use a different template. Please see this post for the Business Page template.
The overall Facebook cover image size is 820 x 360. However not all of that should be used for vital information, as the top and bottom portions are only visible on mobile, while the left and right portions are only visible on desktop. There are also areas on each platform obscured by the profile pictures and buttons.
This template is at 2X scale, so it's 1640 x 720. The reason for that is that you should be designing your cover image at twice-the-size for users with High-DPI displays (most mobile users), even if you don't have one yourself.
Here is what the template looks like (skip to the bottom of the page if you just want the download link, this is a reduced-size version):
When installed as your cover image, here is what a user on a Desktop computer will see (gradient removed or it would be doubled):
Meanwhile, here is what a user on a Mobile device will see (gradient removed as it will not display on Mobile):
As you can see, you should put your most crucial information in the red, near the top and center. Ancillary design can be added in the gradient area.
But, any content in the dead center of the screen in the gradient area may be blocked by the Profile Photo on Mobile. Any content on the left of the screen may be blocked by the Profile Photo on Desktop. Anything in the purple areas will only be seen on Desktop but not Mobile. Anything in the yellow areas will only be seen on Mobile but not Desktop.
It's quite a complicated thing to keep track of. Basically, keep your most important parts of the design in the RED and unobstructed by any of the other design elements you see in the file.
Anyhow, that's how it works. You may download the template by clicking the download button here (59.5 KB).
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