What Is an Ad Impression?

An ad impression occurs when an ad is loaded and displayed to a user. An impression is a metric used to measure how many times that ad has been viewed, then forms the basis for measuring campaign performance. Impressions are one of a few common metrics that are used to calculate payment in web and search advertising, with publishers often charging per-impression or per 1,000 impressions (referred to as Cost per Mille or CPM). Impressions do not measure any type of interaction with an ad, such as clicks, but instead measures each time it has the potential to be viewed by a user.

How ad impressions work

Ad servers provide an invisible image called a pixel to each publisher’s page and can track when a page containing a pixel loads. When a page with a pixel does load, an impression is counted. These numbers are then reported back to the advertiser, which enables them to see how often an ad is being seen. However, because impressions are counted each time content is loaded, the numbers can be skewed by page loads that do not represent an actual new page view. There is also no guarantee that a user loading a webpage actually viewed the ad. Impressions still provide useful data, but advertisers must take these issues into account as they consider how to interpret and act on impression data. Impression data alone cannot always be counted on to measure the success of a campaign.

What is a served impression? 

A served impression, the simplest definition of an impression and the easiest to measure, is counted whenever an ad server receives a request and serves an ad in response. They are easy for advertisers to measure because the data comes from the ad server, meaning no further information from the web page or user is needed. 

What is a viewable impression?

A viewable impression is a metric meant to counteract challenges that come with served impressions. A viewable impression is only counted when at least 50% of an ad is visible on a user’s screen for one second or more. This number is more meaningful to marketers because it represents an actual view of the ad and not just a load of the web page. Ads may be loaded but never seen for many reasons including ad blocking software, screen ratio issues, missing browser plugins, or device incompatibility. Tracking viewable impressions helps alleviate these false impressions and gives marketers more accurate data.  However, unlike served impressions, measuring viewable impressions requires additional data from the user’s device in order to ensure the ad actually appeared on screen.

Why are ad impressions important?

Ad impressions are important for two main reasons; First, they show the reach of an advertiser’s campaign and they provide important data about how many people are seeing an ad. Impressions also form the basis for many important metrics such as click through rate and conversion rate, which help marketers measure the success of a campaign and learn how to improve it over time. Second, impressions are a common method for pricing online advertisements. Impressions provide a fair way to measure the publisher’s effectiveness in displaying ads to their viewers regardless of the ads’ results.

See viewability for additional measurement overlay.