Abstract: This specification describes technologies relating to selection and delivery of online content. One aspect of the subject matter described in this specification can be embodied in methods that include determining a share fraction based on a received reserve price and based in part on a distribution of past bids for content inventory units in one or more content slots provided by a publisher. The methods may further include determining a second reserve price based in part on the received reserve price and based in part on a distribution of past bids for content inventory units in one or more content slots provided by the publisher. The methods may further include receiving one or more bids for the content inventory unit and allocating the content inventory unit to a buyer based in part on the one or more bids and the second reserve price.