Impression Share Calculator
How Much of the Available Audience You Actually Captured
Impression share compares the impressions an ad actually received against the total it was eligible to receive, revealing how much reach was left on the table due to budget limits, bid competitiveness, or ad quality — capacity a campaign could have captured but didn't. A campaign performing well on every other metric can still be leaving significant volume unclaimed if impression share is low.
The Formula
Impression Share = (Impressions Received / Total Eligible Impressions) × 100
Lost Impression Share = 100% − Impression Share
Lost Impression Share = 100% − Impression Share
Where This Calculation Matters
- Diagnosing why volume is lower than expected — low impression share points to budget or bid constraints rather than a demand problem.
- Deciding where to increase budget — a high-performing campaign with low impression share is often the best candidate for additional spend, since it's already proven to convert.
- Competitive positioning — declining impression share over time, with eligible impressions holding steady, usually signals stronger competition entering the auction.
- Budget vs. rank diagnosis — separating impression share lost to budget from impression share lost to ad rank clarifies whether the fix is more budget or a better bid/quality score.
Worked Examples
| Impressions received | Eligible impressions | Impression share | Lost share |
|---|---|---|---|
| 42,000 | 60,000 | 70.00% | 30.00% |
| 18,000 | 60,000 | 30.00% | 70.00% |
How to Use This Calculator
- Enter the impressions the ad actually received.
- Enter the total eligible impressions — the impression volume it could have received had it won every eligible auction.
- Select Calculate to get impression share and lost impression share.
Related Calculations
See what that captured volume cost with the CPM Calculator, or check whether spend is on track with the Ad Budget Pacing Calculator.