“Number Talks” are an increasingly popular instructional routine in K-12 math classrooms.” Developed originally by Kathy Richardson and Ruth Parker and popularized by Sherry Parrish and Jo Boaler, Number Talks are 10-15 minute daily exercises where students practice solving problems without pen or paper. By flexibly solving problems and learning from other students’ strategies, Number Talks are claimed to increase students’ “number sense” (Boaler, 2015; Boaler, Dieckmann, Pérez-Núñez, Sun, & Williams, 2018). However, researchers and educators have not measured this hypothesized growth in student “number sense” in any systematic way. The goal of this proposed project is to develop a measure of student number sense with valid and reliable scores that can be used to assess the impact on student number sense of instructional practices such as Number Talks. Project design and pilot results from a sample of 254 6th-8th grade students will be discussed.