Viewability definition

What is viewability?

Viewability is a metric for ad units that measures how “visible” ads are on a website. It is calculated as a ratio of total viewable ad impressions to the total number of measurable impressions.

Viewability =  (Viewable impressions ÷ Measurable impressions) × 100

According to the definition of the International Advertising Bureau (IAB):

  • Display ads. An impression is considered viewable when at least 50% of the ad pixels were displayed on screen for at least one continuous second.
  • Video ads. An impression is considered viewable when a minimum of 50% of the ad is in view for a minimum of two continuous seconds.
Why is it important?

Sites with high overall viewability are considered a reliable and valuable inventory among advertisers. The importance of high viewability is growing as more and more premium demand partners are looking at how visible their ads can be, rather than CTR.

Sites with low overall viewability are excluded from brand advertisers’ bids, therefore, they have low fill rates and could suffer from ad revenue disruptions in the long run. 

Read more about viewability and why it is important on our blog.

What is the optimal viewability? 

For your ad inventory to be considered reliable and valuable, your site must have at least medium viewability.

A site is considered to have optimal viewability if it exceeds 70%.

Below you will find a table on the different types of site viewability.



Extremely Low 

< 30%






> 70%