Although many educational programs have demonstrated the potential to increase student learning, few examples of successful scaling exist. We study the scalability of a home-based reading program that has shown promising results in an experiment within a local government. Using a nationwide experiment among the full population of 2nd-grade children in Danish public schools (n=51,312), we find that the intervention is less effective at large scale. We provide evidence on potential explanations for the lack of scalability, which suggests that low levels of take-up among both schools and parents were the most important barriers to successful scaling.