What is fill rate in advertising?
Fill rate is the percentage of ad requests that were actually filled with an ad. If a website asks for 1,000 ads and 850 are served, the fill rate is 85%. Publishers track it to see how much of their inventory is selling; low fill can mean weak demand, floor prices set too high or technical problems.
Short answer · AdTech Sumo
Fill rate = ads served ÷ ad requests. It differs from sell-through rate, which measures how much of forecast inventory was sold in direct deals. Unfilled slots may show house ads or stay blank.
A 100% fill rate is not always good: filling every slot by lowering floor prices can attract low-quality demand, including malvertising. Publishers balance fill, eCPM and user experience.