Unfilled Impressions
Ad requests that receive no paid advertisement, resulting in blank ad slots or house ads.
What are Unfilled Impressions?
Unfilled impressions are ad requests from a publisher's site that fail to return a paid advertisement. When an ad slot sends a request to the ad server or demand partners and no advertiser is willing to bid (or all bids fall below the publisher's floor price), the impression goes unfilled. Depending on the publisher's configuration, unfilled slots may show a blank space, a house ad, a public service announcement, or collapse entirely.
Unfilled impressions are sometimes called remnant inventory, though this term more broadly refers to lower-value inventory that is difficult to sell at premium rates. The unfilled impression rate is the inverse of fill rate — if your fill rate is 85%, your unfilled rate is 15%.
Why It Matters for Publishers
Every unfilled impression represents direct revenue loss. If you have a 15% unfilled rate and your filled impressions average $3 eCPM, you're leaving an estimated $0.45 per 1,000 total ad requests on the table — and that adds up quickly at scale. For a site with 1 million monthly ad requests, a 15% unfilled rate at $3 eCPM means approximately $450 in monthly lost revenue.
Unfilled impressions also impact the user experience. Blank ad spaces look unprofessional and can cause layout shifts that hurt Core Web Vitals. Collapsing empty ad slots is better but can make pages look inconsistent.
Tips for Optimization
- Analyze unfilled patterns: Identify where unfilled impressions concentrate — specific geographies, times of day, ad sizes, or page types — and target your optimization efforts accordingly.
- Add backup demand sources: Connect additional SSPs, exchanges, or networks to fill gaps left by your primary demand partners.
- Review floor prices: High floor prices are the most common cause of unfilled impressions. Lower floors on segments with high unfilled rates to capture bids that were previously rejected.
- Use house ads strategically: Fill unsold inventory with promotions for your newsletter, premium content, or affiliate offers to extract some value from otherwise wasted impressions.
- Implement collapse-on-empty: Configure ad units to collapse when unfilled rather than showing blank space. This maintains a cleaner page layout and prevents CLS issues.