Viewability is the share of measured ad impressions that had a real opportunity to be seen by a person. The canonical reference is the Media Rating Council (MRC) Viewable Ad Impression standard: for a display ad, at least 50% of the ad’s pixels must be in the viewport for at least one continuous second; for video, at least 50% of pixels must be in the viewport for at least two continuous seconds. The standard is a measurement floor — it confirms the ad could have been seen, not that anyone looked.
Denominator is measured, not served — the measurement rate itself is a number worth reading separately.
The denominator matters: it is measured impressions, not served impressions. Not every served impression is measurable — cross-domain iframes, ad-blockers, and certain in-app mobile environments can fail to report viewability signals back to the verifying vendor. The measurement rate (measured_impressions / served_impressions) is itself a number worth reading on a vendor report. A vendor reporting 80% viewability on a 60% measurement rate is reporting on a sample, not the full campaign.
Viewability is the gap between what the brand pays for and what could plausibly have been seen by a person. The buyer is billed on served impressions priced at CPM; only a share of those impressions clear the MRC bar. Programmatic display industry-wide routinely runs in the 50–70% viewability range, meaning a meaningful share of CPM spend is buying impressions no human eye encountered. That is the mechanism that makes raw CPM comparisons misleading across placements and platforms. Viewable CPM (vCPM) — the unit cost normalized to viewable impressions rather than served impressions — is the more honest unit when comparing inventory with different viewability profiles.
The verification layer matters because the platform measuring its own delivery has an incentive to report favorably. Third-party vendors (Integral Ad Science, DoubleVerify, MOAT) audit viewability independently of the ad platform, and high-discipline buyers transact on third-party-verified viewable impressions contractually, not on platform-reported numbers. When impression counts look healthy but outcomes don’t, the chain to inspect is served → measured → viewable → verified, in that order.