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Reference library · Cross-media measurement
Viewability
Viewability is the share of served ad impressions that met a defined on-screen exposure threshold, meaning the ad was actually capable of being seen rather than merely loaded. Under the MRC/IAB standard, a display impression counts as viewable if at least 50% of its pixels were on screen for at least 1 continuous second; video requires 50% of pixels for 2 continuous seconds.
Formula
Worked example
A display campaign serves 2,000,000 impressions. The verification tag can measure 1,800,000 of them (the rest fail to load a measurement pixel or run in unsupported environments). Of those measured, 1,224,000 meet the 50%-pixels/1-second threshold. Viewability rate = 1,224,000 / 1,800,000 = 68%. That's below the 70-75% benchmark typically used for premium display, so the buyer flags the placements for review or renegotiates toward a viewable-CPM guarantee.
How it is used
Buyers use viewability to judge inventory and placement quality (above-the-fold vs. below, in-app vs. web, publisher A vs. B) and increasingly to set billing terms, buying on a vCPM so non-viewable impressions aren't paid for. The recurring mistake is dividing viewable impressions by total served impressions rather than by measurable impressions, which silently punishes inventory with poor measurability (e.g. some in-app or AMP environments) even when the impressions that could be measured performed fine.
The common mistake
Always calculate the rate against measurable impressions, not served impressions, or you conflate a measurement-coverage problem with an actual viewability problem.