This paper describes and demonstrates a new online resource developed to help researchers proactively plan for student attrition: the Student Attrition Lookup Tool (SALT). With SALT, researchers can access student mobility estimates for different student and school subpopulations and for different transition periods from kindergarten to grade 12. The paper has three main objectives: (1) describe the data and methods behind SALT; (2) demonstrate how to navigate SALT; and (3) demonstrate how to use student attrition benchmarks in a power analysis. The paper is primarily intended for applied researchers planning school-based studies that follow students over time. SALT is based on four longitudinal data sets from the National Center for Education Statistics (NCES). With the four NCES surveys, the authors constructed analytic files to examine student mobility across six transition periods. Since applied researchers increasingly utilize data from district or state student information systems, the authors estimated three types of student mobility rates (for each transition period) to reflect three different levels of data coverage: (1) any mobility (left baseline school/outside school data); (2) left the district, state, or public school system (outside district data); and (3) left the state or public school system (outside state data, including dropouts). [SREE documents are structured abstracts of SREE conference symposium, panel, and paper or poster submissions.]