Fill rate is an app metric which measures ad network performance. Fill rate is calculated by dividing the number of ad impressions an app shows by the number of times an app has requested an ad from an ad network.
Fill rate = impressions / ad requests
In addition to looking at which network can deliver the highest eCPM, app developers also look at which network can deliver the highest fill rates – since these are the networks which can fill a high percentage of app requests. The more ad requests that are filled, the more ad revenue app developers are able to generate.