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Impression

General

Ad Impression

A single instance of an ad being displayed to a user, regardless of whether it's clicked, the basic unit of measurement for CPM-based advertising.

Definition

An impression is a single instance of an ad being displayed to a user, counted regardless of whether the viewer engages with it in any way. It's the fundamental unit of measurement for reach-based advertising, and the basis for CPM pricing, where advertisers pay per thousand impressions rather than per action taken.

Impressions differ meaningfully from clicks: a campaign can generate millions of impressions while only a small fraction of viewers actually click through, since impressions measure exposure, not engagement. Some platforms distinguish "served" impressions from "viewable" impressions, the latter requiring the ad to actually appear on screen for a minimum time, a more rigorous standard than simply being loaded on a page.

Formula

CPM (Cost Per Mille) = (Total Ad Spend / Total Impressions) × 1,000

Worked Example

A display advertising campaign spends $2,000 and generates 800,000 impressions.

  • CPM: ($2,000 / 800,000) × 1,000 = $2.50

If that same campaign generates 12,000 clicks from those 800,000 impressions, the click-through rate is 12,000 / 800,000 = 1.5%, a separate metric measuring how effectively those impressions converted into engagement.

Key Things to Know

  • Impressions measure reach, not engagement. A large impression count doesn't guarantee anyone actually noticed or acted on the ad, it's a visibility metric first.
  • Viewable impressions are a stricter, more meaningful standard than served impressions. An ad loaded off-screen or scrolled past quickly might count as a served impression but not a viewable one, depending on the platform's measurement standard.
  • Frequency capping limits repeated impressions to the same user. Without it, a campaign can burn through budget showing the same ad to the same person repeatedly rather than reaching new viewers.
  • CPM campaigns are priced entirely around impression volume, unrelated to clicks or conversions. This makes CPM well-suited for brand awareness goals, less suited for direct-response campaigns focused on action.
  • High impressions with low engagement can still signal a problem. Reach alone doesn't validate an ad's effectiveness, pairing impression data with CTR or conversion metrics gives a fuller picture of campaign performance.

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Related Terms

CPMCost Per Mille (Cost Per Thousand Impressions)CTRClick-Through Rate

Frequently Asked Questions

Does an impression require the user to actually see the ad?
Not always in the strictest counting sense, a standard impression counts when the ad is served or loaded, though "viewable impressions" are a more rigorous standard requiring the ad to actually be visible on screen for a minimum duration.
How is an impression different from a click?
An impression is simply the ad being shown, a click requires the user to actively engage with it. Most ads generate far more impressions than clicks, since only a small percentage of viewers click on any given ad.
Why do advertisers care about impressions if they don't guarantee engagement?
Impressions measure reach and visibility, valuable for brand awareness campaigns where the goal is exposure rather than immediate action. [CPM](/glossary/cpm/) pricing is built entirely around this metric for exactly that reason.
Can the same user generate multiple impressions for the same ad?
Yes, if a user sees the same ad multiple times, whether through repeated page visits or ad refreshes, each viewing typically counts as a separate impression, which is why frequency capping exists to limit how often the same user sees an ad.
Is a high impression count always a good sign for a campaign?
Not necessarily on its own, high impressions with low [CTR](/glossary/ctr/) can signal the ad isn't resonating with the audience, impressions measure reach, not effectiveness, so they're best interpreted alongside engagement metrics.