Measurement
Viewability
Viewability measures whether an ad actually had a chance to be seen on screen; the MRC standard is 50% of pixels in view for one second (display) or two seconds (video).
An ad being "served" only means the code ran. It might have loaded at the bottom of a page nobody scrolled to, in a background tab, or behind something else. Viewability asks the more useful question: did the ad appear on the screen long enough to have a chance of being noticed?
The MRC and IAB guideline defines a viewable impression: for display, at least 50% of the ad's pixels in the viewable part of the browser for at least one continuous second; for large display ads (242,500 pixels or more), 30% for one second; for video, 50% of pixels for two continuous seconds. Measurement uses browser geometry and visibility signals, and in apps the Open Measurement SDK.
Viewability is a floor, not a guarantee of attention: a viewable ad can still be ignored. Many advertisers buy on viewable CPM (vCPM) or set viewability targets, and some brands or agencies require stricter thresholds than the MRC minimum. Low viewability on a placement is also a clue to hidden ads, ad stacking or auto-refresh abuse.
Think of it like this
Viewability is like counting a billboard view only if the billboard was facing the road while cars drove past, not while it was covered by a tarpaulin.
An example
A campaign in France serves 2 million display impressions; 1.3 million meet the MRC standard, a 65% viewability rate. Buying on vCPM at €4, the advertiser pays only for the 1.3 million viewable ones.
Related terms
Viewable impression
A viewable impression is an ad impression that meets a viewability standard, by default the MRC's: 50% of the ad's pixels on screen for one continuous second, or two seconds for video.
MRC (Media Rating Council)
The MRC (Media Rating Council) is a US non-profit industry body that sets measurement standards and audits and accredits media measurement services, including viewability and invalid-traffic filtering.
Attention metrics
Attention metrics estimate how much people actually noticed or engaged with an ad, using signals like time in view, screen share, interaction and eye-tracking panels, beyond simple viewability.
Open Measurement SDK (OM SDK)
The Open Measurement SDK (OM SDK) is an IAB Tech Lab standard toolkit that lets verification vendors measure viewability and invalid traffic inside apps and other environments through one shared integration.
Ad verification
Ad verification is independent third-party measurement that checks whether ads were served to real people, in viewable positions, in safe and suitable environments, and in the intended geographies.
Sources: MRC Viewable Ad Impression Measurement Guidelines, Google Ad Manager Help: Active View measurement