What is viewability in advertising?
Viewability measures whether an ad actually had a chance to be seen. Under the Media Rating Council standard, a display ad counts as viewable when at least 50% of its pixels are on screen for at least one continuous second; for video, 50% of pixels for two continuous seconds. Many advertisers now buy or optimize on viewable impressions only.
An ad can load (and be billed as an impression) at the bottom of a page no one scrolls to. Viewability closes that gap. Large display ads (over 242,500 pixels) need only 30% on screen. The share of measured impressions meeting the standard is the viewability rate, and a viewable impression is the unit some advertisers pay for.
Viewability is a minimum, not proof of attention: an ad can be viewable for one second while a person ignores it. That is why attention metrics emerged. Low viewability also flags problems such as ad stacking, pixel stuffing and ad-cluttered made-for-advertising sites pages.