Learning by doing refers to learning practices that involve completing activities as opposed to explicit learning (e.g., reading). Although the benefits of learning by doing have been described before, it is still relatively uncommon in instructional practice. We investigated how much students employ learning by doing in online courses, and whether it is associated with improved learning outcomes. Spending more time completing activities had a larger impact on learning outcomes than spending more time reading, even in the case of mostly declarative content, such as in a Psychology course. Moreover, learning by doing is more efficient: grade improvements of 1 standard-deviation require 10-20% less time in learning by doing than reading. Finally, we contrast this evidence with students’ a priori intuitions on best study strategies for their online course. Students overestimate the value of explicit learning through reading, and underestimate the value of active learning.