Definition & Explanation

Viewability is an ad tech metric that measures whether an ad was actually seen by a human user. The MRC and IAB standard defines a display ad as viewable when at least 50% of its pixels are visible in the browser viewport for one or more continuous seconds (two seconds for video ads). This metric emerged as a response to the discovery that a significant portion of served ads were never actually seen, helping the industry shift from paying for impressions served to paying for impressions viewed (vCPM).