Viewability
Viewability measures whether a served ad was actually in view, based on how much of it entered the screen and for how long.
Viewability is a measure of whether a served ad actually had the chance to be seen, based on how much of the ad entered the user's screen and for how long. It exists because a counted impression and a seen impression are not the same thing.
The widely used IAB standard defines a display ad as viewable when at least 50% of its pixels are in the viewport for at least one continuous second; for video, the threshold is 50% of pixels for two continuous seconds. Larger formats often have stricter rules.
Why it matters#
Many ad impressions are served but never seen because they load below the fold, in a background tab, or in a slot the user scrolls past too quickly. Viewability separates inventory that had a real chance to be seen from inventory that didn't, so advertisers don't pay full price for invisible ads.
Because CPM (Cost Per Mille) bills per thousand served impressions, low viewability quietly inflates your true cost per seen impression. A 70% viewability rate means roughly a third of what you paid for was never on screen. Many buyers now negotiate viewable-CPM (vCPM) terms or set minimum viewability thresholds to protect spend.
Viewability is also a brand safety and quality signal: high-viewability placements tend to be premium, above-the-fold, and less associated with fraud or low-quality stacked-ad inventory.
Related terms: Ad Impression, CPM (Cost Per Mille), and Brand Safety.
