Glossary
Fill rate
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Fill rate is the share of ad requests that end with an ad being served, calculated as filled impressions divided by ad requests for a placement, site, app or demand partner.
Every time a page or app asks for an ad, that is a request. If no buyer bids above the floor, or the winning creative fails to load, the slot stays empty or shows a house ad. Fill rate tells a publisher how much of its available inventory turns into paid impressions.
Low fill has a few usual causes: floors set above what buyers will pay, too few demand partners, traffic that buyers value little, timeouts, and filtering of suspected bots. In OpenRTB, a bidder that passes can return an empty HTTP 204 response, or a response with a no-bid reason code in the nbr field. Those codes help the seller see why requests go unfilled (IAB Tech Lab, OpenRTB 2.6 implementation guide, section 7.1).
Fill rate must be read with price. Lower floors raise fill but can cut revenue per impression, so publishers watch fill rate and eCPM together. A publisher case study on fill rate shows the trade-off in practice.
Example: A mobile app sends 10,000,000 ad requests in a month and serves 7,200,000 paid impressions, a 72% fill rate. Raising the floor from $1.00 to $1.40 cuts fill to 61% but lifts eCPM, so the publisher compares total revenue before keeping the change.
Related terms: ad network, header bidding, floor price, eCPM, yield management.