X and Twitter header size: 1500 by 500

Three to one is an extreme shape and it behaves like one. There is very little vertical room, so a photograph with a subject in it almost never works: the subject either fills the strip and loses its context, or sits small in a band of background. Banners that hold up at this ratio are usually pattern, texture, gradient or type set deliberately wide.

The avatar overlaps the lower left corner of the strip and is drawn as a circle on top of it. Anything placed there is hidden, and the overlap is proportionally larger on a narrow screen than on a wide one, so a logo tucked into that corner disappears exactly where most people will see the profile.

The banner is also cropped rather than letterboxed as the viewport changes. A narrower screen shows less of the width, taken from the middle, and a screen with a different aspect can trim the height. The reliable strategy is a design whose meaning survives losing a third of its width, which in practice means centring anything that must be read.

There is no 3:1 preset in the cropper, whose fixed ratios are 1:1, 4:3, 16:9, 3:2, 2:3 and 9:16. Enter 3 and 1 as a custom ratio, or 1500 and 500 as explicit dimensions, and the calculator will confirm the shape before you export.