What is Viewability?

Viewability is the share of ad impressions a user could actually see — not only the ones the ad server delivered. MRC (Media Rating Council) and IAB set the bar: for display, at least 50% of the creative’s pixels must be in the viewable area of an in-focus tab for a continuous ≥1 second; for video, ≥50% of pixels for a continuous ≥2 seconds. Large units (≥242,500 CSS pixels) count as viewable at ≥30% in-view for 1 second. An ad served below the fold, in a background tab, or flashed during scroll does not meet that bar.

Viewable impression vs served impression

A served impression (sometimes rendered / ad-server impression) means the creative was requested and delivered into the slot. CPM billing on the open auction and in many networks still uses served counts unless the deal is priced on viewable CPM (vCPM).

A viewable impression is a served impression that also clears the MRC/IAB threshold. The gap is familiar: the slot exists in the DOM, the tracking pixel fired, the user never saw the banner. Typical causes include below-the-fold placements without a scroll, background tabs, instant interstitial closes, a player outside the viewport, or another layer covering the creative.

Reporting formula: viewability rate = viewable impressions / measurable impressions. Some impressions are unmeasurable (no JS measurement, tag failure, a cross-domain iframe without access). Those are not dumped into the denominator as “unviewable,” or the publisher looks worse than the inventory is. Buyers read three numbers — served, measurable, viewable — and only then CTR and post-click CR.

Why 50% / 1–2 seconds

The threshold is an industry compromise, not “attention” and not a completed view. The user may have looked away; MRC records geometry and time in the viewport. Some buyers (historically GroupM and large agencies) require 100% of pixels in-view — that is a deal term, not the base standard. Video still reports quartiles and completes separately: viewable ≠ watched to the end.

Do not mix viewability with view-through conversion. VTC credits a conversion after an impression with no click. A viewable impression may produce no VTC; some UIs still attach VTC to served impressions if visibility is not filtered. In DSP reports and Google Active View those are different columns.

Where measurement breaks

On display and out-stream video, viewability drops on long articles, mobile web with sticky units, and in apps where the SDK records a serve before the slot is on screen. Prefetch and lazy load inflate served counts. Bot traffic and click fraud create “impressions” with no person in the viewport — a verification pixel may mark them unviewable or invalid, but not always.

Measurement usually sits in third-party tags (Google Active View, IAS, DoubleVerify, Moat, and peers): a script in the creative or on the page computes intersection with the viewport. A 5–15% gap between ad server and verifier is common because “measurable” definitions and timeouts differ. Compare sources on the same tag, not DSP UI versus publisher UI.

Buying and arbitrage

In programmatic, buyers set a minimum viewability pre-bid (skip the auction if the forecast is below the floor) or filter post-bid. vCPM deals pay only for viewable impressions; open CPM is cheaper, but part of spend lands in unseen slots. High viewability on MFA (made-for-advertising) sites does not imply CR: the banner is visible, intent is zero, bounce is high.

In performance and arbitrage, viewability is inventory hygiene, not an offer KPI. Read it next to post-click CR and EPC in the tracker: a low CPM at 20% viewability already costs as much per visible contact as a “expensive” domain. On retargeting, an unseen impression does not warm the audience but can still burn frequency. In-app uses the same idea; the SDK, not a browser viewport, does the count.

What the metric does not say

Viewability is not attention, not brand lift, and not creative policy. Fifty-one percent of pixels for one second is a formal pass. Time-in-view beyond the threshold, share of screen, and video completion are separate columns if the buyer pays for them. Served CPM, vCPM, and eCPM are not interchangeable: the denominator is a different unit.