Abstract: Systems and methods for optimizing impression frequency for content presented on a client device are provided. A content distribution system selects a subset of available impressions on which to place bids such that a predetermined content distribution budget is spent and the bids are placed on impressions associated with a minimum number of unique user identifiers. The content distribution system receives a notification of an available impression for presenting content on a client device and identifies a user identifier associated with the client device. The content distribution system calculates a hash value for the user identifier as a function of the user identifier and compares the hash value with an impression probability threshold. The content distribution system determines whether to bid on the available impression based on a result of the comparison.