Fill rate is a term that refers to the number of ads served divided by the number of ads requested.
For example, if you requested 1000 impressions of your ad from the publisher, but of this number, only 250 impressions were realized, then your Fill rate is 25%. This process can be represented in the form of a simple formula:
Fill Rate = Total Ad Impressions/Total Ad Requests X 100%
Why is this happening? The process of selling advertising space on the site occurs in real-time in the auction mode. Accordingly, advertisers compete for each client. But there are situations when clients do not meet advertisers’ requests and ads are simply not shown for them. For example, advertisers are willing to pay more for US users than for Indian viewers. And if a person with geolocation from Asia comes to the site, the cost of an impression for him decreases significantly. This situation is not suitable for advertisers or publishers who do not want to lower the bar on the quality and price of their ad space. As a result, a user from India will most likely simply not see the ad and will lower the Fill Rate.
Often publishers do not want to deal with this problem, arguing that a decrease in the price of an impression for an individual client can adversely affect the overall quality of advertising because the advertiser was not paying for this.