Ad Impression

An ad impression is a single instance of an ad being served and rendered on a page, feed, or app where a user could see it. One impression equals one display of one ad to one user; if the same ad loads three times, that counts as three impressions.

Impressions are the base unit of ad exposure and the denominator behind many performance metrics. They measure reach and frequency: how many times your ads were shown, not how many people acted on them.

How it works and how it's billed#

An impression is typically counted when an ad is requested from the ad server and rendered in the page. This is the basis for CPM (Cost Per Mille) pricing, where advertisers pay per thousand impressions served. It's also the foundation of CTR (Click-Through Rate), which divides clicks by impressions.

A served impression isn't a seen impression#

A crucial caveat: an ad can be served and counted as an impression even if it loads below the fold and the user never scrolls to it. That gap is exactly what viewability addresses, a stricter standard that counts an impression only when enough of the ad actually entered the viewport for long enough. Smart buyers track both raw impressions and viewable impressions, because paying for ads no one sees is wasted spend.

Related terms: CPM (Cost Per Mille), Viewability, and CTR (Click-Through Rate).