Fill Rate

Fill rate is the percentage of ad impression opportunities (ad requests) that are fulfilled with an actual ad. It is calculated as: (Filled Impressions ÷ Total Ad Requests) × 100. A fill rate of 100% means every ad slot in every email that was opened had an ad delivered. A fill rate of 0% means no ads were served. In newsletter advertising, fill rate depends on advertiser demand, floor CPM settings, category block rules, and programmatic demand availability. Publishers on MailAdx typically achieve 90–97% fill rates once their waterfall (direct-sold + programmatic + house ads) is properly configured. Unfilled impressions represent lost revenue — fill rate optimization is one of the highest-ROI activities for newsletter publishers.