What is an ad impression and how is it counted?
How an ad impression is counted, how it differs from viewable impressions and reach, and why counts differ between systems.
Written by Samhub with the help of AI·Updated 13 Oct 2026·3 min readAn ad impression is one ad shown once. Advertising is often priced per thousand impressions (CPM).
Impression, viewable impression and reach
| Metric | Counts |
|---|---|
| Impression | The ad was served or began to render |
| Viewable impression | The ad was at least 50% visible for one second (two for video) |
| Reach | Unique people who saw the ad |
Why counts differ
The ad server, the buyer’s platform and verification tools count at slightly different moments and filter invalid traffic differently. Small gaps are normal; large ones need investigating.
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From impressions to value
Revenue = impressions × CPM ÷ 1,000
Example: 2,000,000 × €3.00 ÷ 1,000 = €6,000
Illustrative numbers.
The price per impression depends on what the buyer knows about the reader. Audience data is what turns the same impression into a higher price.
CPM uplift at Stampen Media after activating first-party audiences with Samhub.
Frequently asked questions
No. An impression can be counted even if the reader never scrolls to the ad.
An impression counted when the ad server sends the ad.
Mostly per thousand, as CPM.
They should be filtered out as invalid traffic.
Related terms
The price of 1,000 ad impressions.
Whether an ad had a real chance to be seen.
How many ads were shown versus how many people saw them.
Impressions and clicks not made by real, interested people.
The share of ad views that lead to a click.
Limiting how often one person sees an ad.
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