In April 2013, Results for Development Institute (R4D) reviewed the benefits of primary education and estimated the economic cost associated with large populations of out-of-school children in a background study for Educate A Child’s (EAC) High Level Strategic Meeting to Accelerate Efforts to Reach Out-of-School Children (Burnett, Guison-Dowdy and Thomas 2013). This report is an extension of that study. It updates economic cost estimates to reflect the latest data from the UNESCO Institute for Statistics (UIS), further develops the estimation methodology, and expands the estimation exercise to a set of 20 low- and middle-income countries.