Techniques for estimating advertisement inventory for search retargeting segments are provided. A total inventory for each cluster of a plurality of clusters is estimated based on a history of previously viewed pages and previously used search keywords that were submitted by users is searches performed prior to accessing the previously viewed pages. A search retargeting (SRT) segment is received that includes a set of search keywords. The SRT segment is received in a request for deliverable advertisement inventory for advertisements associated with the SRT segment. A deliverable inventory for the received SRT segment is estimated based on the estimated total inventories for the clusters.