Ad viewability

Ad viewability measures whether a served impression actually had the opportunity to be seen, rather than simply loading somewhere on a page. The widely used Media Rating Council standard defines a display ad as viewable if at least 50% of its pixels were on screen for a minimum of one continuous second, with a stricter two-second threshold for video, and different standards can apply for larger ad formats.

Viewability is distinct from an impression being counted at all: an ad can load in a browser tab that is never scrolled to, sit behind another window, or render in a background tab, and still register as a served impression without meeting any viewability standard. It is also distinct from invalid traffic, which concerns whether the traffic was a real human at all; an impression can be technically viewable to a bot, or genuinely unviewable but still served to a real, engaged person.

Viewability matters because it is the baseline precondition for an ad to have any chance of influencing someone, which is why it underpins credible brand lift and view-through conversion measurement, and it is increasingly used as a pricing basis, with advertisers paying only for viewable CPM. Publishers and ad placements vary widely in typical viewability rates by format and position on the page, which is why buyers set minimum viewability thresholds into their campaign targets rather than assuming all inventory performs the same.