1. A computer implemented method comprising: identifying a campaign including one or more targeting criteria for targeting content items to users; determining, by one or more processors, a number of unique users that will be targeted based on the targeting criteria in a predetermined time period including: identifying historical auction results including content served in response to content requests; storing the historical auction results as a plurality of documents, one for each auction result, wherein storing an auction result as a document comprises storing serving criteria and a user identifier for a user that was served a content item based on the auction result; evaluating the plurality of documents including initiating a first query on the plurality of documents to determine an estimated number of impressions that would occur in the plurality of documents based on the targeting criteria including generating a first count for occurrences in the plurality of documents that satisfy the first query including identifying a location in the plurality of documents wherein the first count satisfies a first threshold number of occurrences; determining an estimated total number of impressions for the targeting criteria in the time period based at least in part on the estimated number of impressions in the plurality of documents; and determining a total number of unique targeted users including initiating a second query starting at a next entry after the location in the plurality of documents including estimating the number of unique users for the targeting criteria based on the second query including determining a frequency of impression for a plurality of the users represented in the plurality of documents as a sum of a frequency of impression for a selected number of users divided by the selected number of users and determining the total number of unique targeted users as the total number of impressions divided by the frequency of impression.