Viewability rate
Viewability rate is the percentage of measured ad impressions that qualified as viewable under an industry standard, rather than merely served. The IAB and MRC define a viewable display impression as at least 50% of the ad's pixels in view for at least one continuous second (two seconds for video). The rate exposes the gap between ads delivered and ads actually given a chance to be seen.
What this means
Viewability rate = viewable impressions ÷ measured impressions. It quantifies how often an ad that was counted actually entered the user's viewport long enough to qualify. The complement — impressions that never met the bar — represents exposure paid for but not realistically seen.
The IAB/MRC standard
The widely used standard comes from the IAB and the Media Rating Council: a display ad is viewable when at least 50% of its pixels are in the viewport for at least one continuous second; a video ad requires 50% of pixels for at least two continuous seconds. Large display formats have their own thresholds. Not every impression is even measurable — ad slots in cross-domain iframes can be unmeasurable — so viewability rate is computed over measured impressions, and that base matters when comparing platforms.
- Display: 50% of pixels for ≥1 continuous second
- Video: 50% of pixels for ≥2 continuous seconds
- Rate is over measured impressions, not all served ones
How it appears in analytics and logs
A viewability rate tells you what fraction of impressions met the on-screen standard. A low rate means many impressions were served below the fold or scrolled past too fast to count, regardless of how the buy was priced.
Diagnostic use case
Use viewability rate to judge how much of your bought or sold inventory was actually on-screen, separating served impressions from impressions that had a chance to be seen.
What WebmasterID can help detect
WebmasterID can record first-party element-visibility events using the Intersection Observer API, so on-page 'was it seen' signals use a definition you control rather than an opaque ad-server one.
Common mistakes
- Assuming a served impression was viewable.
- Comparing viewability rates with different measurable bases.
- Applying the display threshold to video impressions.
Privacy and accuracy notes
Viewability is measured from on-screen geometry and timing, not from identifying individuals. It needs no personal identifiers to compute.
Related pages
- Impressions and the viewability problem
An impression counts a piece of content being shown — a search result, an ad, a social post. The trap is that 'shown' has no single definition: Search Console counts a listing appearing in results, ad servers count an ad being delivered, and the IAB/MRC viewable-impression standard requires a portion of pixels visible for a minimum time before it counts. Impressions are only comparable within one definition.
- Cost per mille (CPM)
Cost per mille (CPM) is the cost of one thousand impressions — 'mille' is Latin for thousand. It is the standard pricing unit for awareness and display buying, where advertisers pay for exposure rather than clicks. CPM depends entirely on how an impression is defined (served vs viewable), and it says nothing about whether anyone clicked or converted, so it is an exposure-cost metric only.
- Video completion rate
Video completion rate is the percentage of video plays that reached the end. It is usually built from quartile progress events — playback milestones at 25%, 50%, 75%, and 100% — so completion rate is the 100% milestone divided by starts. It signals whether content holds attention, but auto-play, muting, and background tabs can inflate completions that no one actually watched.
- Event Explorer
Record first-party element-visibility events.
Sources and verification notes
- IAB/MRC — Viewable ad impression measurement guidelines
- MDN — Intersection Observer APIThe browser API used to measure element visibility first-party.
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.