Developmental foundations of children’s fraction magnitude knowledge

•We tested 8th and 9th graders’ understanding of the fractions number line.•Fractions number line performance predicted high school algebra achievement.•Innovative Baysian analyses identified earlier-grade predictors of fraction number line performance.•Conceptual understanding of fractions in 7th grade predicted later fraction number line performance.•Fractions knowledge and whole number knowledge are gradually integrated. The conceptual insight that fractions represent magnitudes is a critical yet daunting step in children’s mathematical development, and knowledge of fraction magnitudes influences children’s later mathematics learning including algebra. In this study, longitudinal data were analyzed to identify the mathematical knowledge and domain-general competencies that predicted 8th and 9th graders’ (n=122) knowledge of fraction magnitudes and its cross-grade gains. Performance on the fraction magnitude measures predicted 9th grade algebra achievement. Understanding and fluently identifying the numerator-denominator relation in 7th grade emerged as the key predictor of later fraction magnitudes knowledge in both 8th and 9th grades. Competence at using fraction procedures, knowledge of whole number magnitudes, and the central executive contributed to 9th but not 8th graders’ fraction magnitude knowledge, and knowledge of whole number magnitude contributed to cross-grade gains. The key results suggest fluent processing of numerator-denominator relations presages students’ understanding of fractions as magnitudes and that the integration of whole number and fraction magnitudes occurs gradually.