Student agency is an important dimension of classroom literacy learning. Recent calls have emphasized the need to support student agency in literacy. Despite this interest, the field lacks a viable instrument that can be used to measure student agency with elementary students. The authors share the Student Agency Profile, a survey instrument that can measure student agency in literacy contexts. The authors provide insight into the validation process, which included 1,794 elementary students, and direction on how to administer, score, and interpret results. A central aim is to show how the survey can be used in practice while supporting teachers in their efforts to cultivate student agency in today's diverse classrooms.