Previous research provides evidence that developing a growth mindset--believing that one's capabilities can improve--promotes academic achievement. Although this phenomenon has undergone prior study in a representative sample of ninth graders in the United States, it has not been studied in representative samples of other grade levels or with standardized assessment measures of achievement rather than more subjective grades. Using a rich longitudinal data set of more than 200,000 students in Grades 4 through 7 in California who we followed for a year until they were in Grades 5 through 8, this article describes growth mindset gaps across student groups and confirms, at a large scale, the predictive power of growth mindset for achievement gains. We estimate that a student with growth mindset who is in the same school and grade level and has the same background and achievement characteristics as a student with a fixed mindset learns 0.066 SD more annually in English language arts, approximately 18% of the average annual growth or 33 days of learning if we assume learning growth as uniform across the 180 days of the academic year. For mathematics, the corresponding estimates are 0.039 SD, approximately 17% of average annual growth or 31 days of learning.