What is an Impression?
An impression is counted each time an ad is fetched and displayed on a user's device. It's the most fundamental unit of measurement in digital advertising, representing one opportunity for a user to see an ad.
Impressions are the basis for CPM (Cost Per Mille) pricing, where advertisers pay per thousand impressions served.
Types of Impressions
| Type | Definition | Use Case |
|---|---|---|
| Served Impression | Ad delivered to the page | Basic counting |
| Viewable Impression | Ad actually seen by user | Quality measurement |
| Verified Impression | Confirmed human view | Premium campaigns |
| Unique Impression | First view per user | Reach measurement |
MRC Viewability Standards
The Media Rating Council (MRC) defines viewability standards that determine when an impression counts as "viewable":
- Display Ads: 50% of pixels visible for at least 1 second
- Video Ads: 50% of pixels visible for at least 2 consecutive seconds
- Large Display (242,500+ pixels): 30% visible for 1 second
Impression vs. Related Metrics
| Metric | What It Measures | Relationship |
|---|---|---|
| Impression | Ad served | Base metric |
| Reach | Unique users | Impressions ÷ Frequency |
| Frequency | Views per user | Impressions ÷ Reach |
| Click | User engagement | Impressions × CTR |
Pro Tip: Quality Over Quantity
Not all impressions are created equal. A viewable impression from an engaged user is worth far more than a non-viewable impression that loads below the fold and is never seen.
Impression Fraud Types
- Ad Stacking: Multiple ads layered on top of each other
- Pixel Stuffing: Ads rendered at 1x1 pixel size
- Bot Traffic: Non-human automated impressions
- Domain Spoofing: Fake sites mimicking premium publishers
- Ad Injection: Unauthorized ads inserted into pages
Impression Counting Methods
- Server-Side: Counted when ad server delivers the ad (less accurate)
- Client-Side: Counted when ad loads in browser/app (more accurate)
- Viewability-Based: Counted only when viewability criteria met (most accurate)
Impression Benchmarks by Format
| Ad Format | Avg Viewability | Avg CPM |
|---|---|---|
| Banner (320x50) | 45-55% | $0.50-2 |
| Interstitial | 85-95% | $8-15 |
| Rewarded Video | 95-99% | $15-40 |
| Native Ad | 60-75% | $3-8 |