When creating a smart audience, the “Estimated Audience Size” section displays an estimate of the number of users who meet the specified criteria, calculated using the “Calculate Audience Size” button.
This estimate provides an indication of how broad or how narrow a target audience is before you save the target audience permanently, which helps you adjust the criteria if necessary.
An estimated size that is much smaller than expected usually indicates that the criteria you’ve set are too strict or too specific, while an unexpectedly large estimated audience may indicate that the criteria are too broad or lack sufficient distinction.
By reviewing this estimate before actually saving the target audience, you avoid discovering only afterward—when you’re using the target audience in a campaign—that the segment doesn’t have the desired size or precision.