Banner ad sizes

The IAB standard sizes, of which a handful carry almost all the inventory: the medium rectangle at 300 × 250, the leaderboard at 728 × 90 and the mobile banner at 320 × 50.

The IAB publishes a long list and the market ignores most of it. Inventory concentrates heavily in a few shapes because those are what publishers actually template into their layouts, and building the tail sizes rarely repays the effort. The billboard at 970 × 250 is the exception worth knowing: it is a premium above-the-fold placement on larger sites, sells for more, and is a genuinely different design problem from the small formats. Every size shares the 150 KB weight limit, which is the real constraint on what these can look like.