Fill rate is the percentage of ad requests that are successfully filled with a paying ad, out of the total number of ad requests a publisher sends. It's a fundamental health metric for publishers: a high fill rate means most available impressions are being monetized, while a low fill rate means inventory — and revenue — is going to waste. Fill rate is calculated by dividing the number of filled (served) impressions by the total number of ad requests, then multiplying by 100. Send 100,000 requests and fill 85,000, and your fill rate is 85%.
 

Every unfilled request represents a missed monetization opportunity. When a user loads a page or app and an ad slot generates a request, but no advertiser bid clears or no demand was available, that slot goes empty (or shows a default/house ad earning nothing). Maximizing fill rate is therefore a core objective of publisher monetization — though, importantly, fill rate must be balanced against price. A 100% fill rate at rock-bottom CPMs can earn less than an 80% fill rate at strong CPMs, which is why publishers optimize for revenue (often measured as eCPM) rather than fill rate in isolation.
 

Several factors influence fill rate. The breadth and quality of demand connections matter most — more competing buyers means more requests get filled. Geography plays a big role: Tier 1 markets like the US, UK, and Germany attract dense demand and high fill, while less-targeted regions may see lower fill. Ad format, placement, floor prices, and audience quality all affect how readily impressions are bought. Setting floor prices too high suppresses fill; setting them too low fills inventory cheaply.
 

Header bidding is one of the most effective tools for improving fill rate. By letting multiple demand sources bid simultaneously rather than in a sequential waterfall, header bidding exposes each impression to maximum competition, increasing the chance it gets filled — and at a better price. Connecting to an ad exchange, which provides per-impression demand from many buyers, similarly helps publishers fill remnant inventory and push fill rate toward 100%.
 

A subtlety worth noting: a reported fill rate of 100% doesn't always mean 100% of value is captured. Ad blockers, for instance, can prevent ads from rendering even on "filled" requests, so the realized value may be lower than the headline fill figure suggests.
 

For publishers, fill rate is a key diagnostic — a low number signals weak demand, mispriced floors, or poor inventory quality that needs attention. Optimized alongside eCPM and viewability, a healthy fill rate ensures that the traffic a publisher works hard to attract is actually converted into revenue.