This study investigates the developmental relationship between whole number and fraction number line estimation, emphasizing the critical role of strategy use and cognitive resources. Using a sample of fifth- and sixth-grade students, the study explores how reference-point-based strategies differ across educational stages and how whole number knowledge, spatial working memory, and arithmetic skills predict fraction estimation accuracy. The results reveal significant developmental differences, with sixth graders demonstrating greater reliance on endpoints and a broader range of strategies compared to fifth graders. Regression analyses underscore the predictive value of whole number estimation accuracy and arithmetic skills, particularly for fraction estimation linearity and accuracy. Spatial working memory also emerged as a key factor in fraction estimation, suggesting its importance in mental segmentation and localization. These findings contribute to understanding the interplay between whole and fraction number representations and offer practical implications for instructional strategies to enhance numerical cognition.