Viewability
A measure of whether an ad had the opportunity to be seen, according to an agreed standard.
What is viewability?
Viewability measures whether an ad was positioned on screen in a way that gave a person the chance to see it. The widely used Media Rating Council guideline counts a display ad as viewable when at least half of its pixels are in view for one continuous second, and a video ad when at least half is in view for two continuous seconds, with larger formats subject to stricter rules. Viewability is measured by verification code or platform tools and expressed as the share of measured impressions that met the standard. It is a prerequisite for attention, not proof of it, and is often used alongside brand-safety and invalid-traffic checks.
How viewability works
Measurement code records how much of the ad was on screen and for how long.
- Measurement code runs with the ad and tracks its position relative to the visible area of the screen.
- It records how much of the ad is in view and for how long.
- Each measured impression is classified as viewable or not against the agreed standard.
- Results are reported as a viewability rate and can be used for optimisation or billing on viewable impressions.
Why it matters
An impression that was never on screen cannot have had an effect. Buyers increasingly pay on viewable impressions.
For advertisers
Paying for or optimising towards viewable impressions reduces spend on ads that had no chance of being seen.
For publishers
Page layout, ad placement and loading behaviour affect viewability, which in turn influences the prices buyers will pay.
Practical example
A banner at the bottom of a long page counts as served but not viewable if the reader never scrolls to it.
Key considerations
- Different vendors can report different rates for the same campaign, so buyers and sellers should agree on one measurement source.
- Some environments, such as certain in-app and CTV placements, are harder to measure consistently.
- Chasing viewability alone can favour cluttered pages with sticky units; it is best weighed against attention and outcomes.
More Measurement & Optimization terms
Frequently asked questions
What is the standard definition of a viewable impression?
Under the MRC guideline, half of a display ad’s pixels in view for one continuous second, or half of a video ad for two continuous seconds.
What is vCPM?
Cost per thousand viewable impressions, a pricing model in which buyers pay only for impressions that met the viewability standard.
How can publishers improve viewability?
By placing ads where readers actually look, using lazy loading so ads load as they come into view, and avoiding slots that are rarely scrolled to.