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User Acquisition

Ad Impression

Also known asImpressionAd View

A single instance of an ad being served to a user — the atomic unit of ad inventory. What counts as "served" vs "viewable" vs "verified" matters enormously for pricing.

Key takeaways

  1. 01An impression = one instance of an ad being rendered to a user. The unit underneath CPM, eCPM, and most ad pricing models.
  2. 02Three impression types: **served** (sent to device), **viewable** (>50% pixels on screen for >1 sec by MRC standard), **verified** (third-party verified).
  3. 03Viewability typically runs 50-75% of served impressions — half of "impressions" never actually meet the standard.
  4. 04Premium advertisers buy only verified impressions; programmatic typically buys served — explains a chunk of the eCPM gap.

An ad impression is a single instance of an ad being rendered to a user — the atomic unit underneath CPM, eCPM, and most digital ad pricing. But "impression" is a surprisingly fuzzy word. The same campaign measured by three different counting standards can produce three substantially different impression counts and three different effective prices.

Three impression types that matter

  1. Served impressions — the ad request was sent to the device. The ad may or may not have rendered, may or may not have been visible. Highest count, lowest standard.
  2. Viewable impressions — by the Media Rating Council (MRC) standard, ≥50% of the ad's pixels were on screen for ≥1 second (≥2 seconds for video). This is what "an impression" generally means in modern brand-side conversations.
  3. Verified impressions — third-party verified (IAS, Moat, DoubleVerify, Pixalate). Premium advertisers buy only verified inventory because they don't trust publisher-reported impression counts.

Why this matters for pricing: a CPM that looks low may be priced against served impressions but include 30% non-viewable inventory. A CPM that looks high may be priced against verified-viewable impressions only. Always know which standard your inventory is being priced and reported against — comparing them at face value misleads you about actual unit economics.

For publishers: eCPM is normally reported against served impressions in mediation dashboards (AppLovin MAX, IronSource, Admob), but the advertiser-facing pricing may be against viewable impressions. The gap is sometimes substantial.

Impression frequency caps: a separate but related concept. Most ad platforms let you cap how many impressions of the same campaign one user can see in a defined window (e.g., 3 impressions per user per 24 hours). Frequency caps prevent ad fatigue but also reduce reach. Tune them based on creative count and audience size — high creative count + small audience = aggressive cap; low creative count + large audience = looser cap.

Quick answers

What counts as an ad impression?

Depends on the standard. **Served impression**: the ad request was sent to the device. **Viewable impression** (MRC standard): ≥50% of the ad's pixels were on screen for ≥1 second (≥2 seconds for video). **Verified impression**: third-party verified by IAS, Moat, DoubleVerify, or Pixalate. The three counts can differ by 25-50% on the same inventory.

What is viewability in mobile advertising?

**Viewability** is whether an ad actually had a chance to be seen. By the Media Rating Council (MRC) standard, an ad is "viewable" if ≥50% of its pixels were on screen for ≥1 second (or ≥2 seconds for video). Roughly 50-75% of served impressions hit viewability — the other 25-50% never do (off-screen, scrolled past too fast, rendered behind other UI).

Why are some advertisers willing to pay much more per impression?

Several reasons stack. (1) They buy verified inventory only, where every impression actually met viewability. (2) Premium ad slot (post-onboarding interstitial, level-completion rewarded video, premium content placement) — much higher attention. (3) Targeted audience — they pay more for a verified-viewable impression of the right user than for unfiltered random reach. The headline CPM gap between low-end and premium mobile inventory can be 50-100× for the same nominal "impression".

Should I use frequency capping on my campaigns?

Almost always yes. Most ad platforms let you cap how many impressions of the same campaign one user sees in a defined window (3 per user per 24h is common). Caps prevent ad fatigue (same user seeing the same creative 20 times → click-through-rate craters → eCPM drops). The right cap depends on creative count and audience size: many creatives + small audience → aggressive cap; few creatives + large audience → looser cap.

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