To test the hypothesis that a higher tendency to "s"pontaneously "f"ocus "o"n "m"ultiplicative "r"elations (SFOR) leads to improvements in rational number knowledge via more exact estimation of fractional quantities, we presented sixth graders (n = 112) with fraction number line estimations and a novel task in which numerical information embedded in narratives could be estimated as fractions. Consistent with our main hypothesis, we found that SFOR tendency predicted both forms of fraction estimation. However, the relation between SFOR and fraction magnitude comparisons was mediated by fraction estimation, both on the number line and on whole number relations embedded in narrative vignettes. Thus, a higher tendency to recognize multiplicative relations in non-explicitly mathematical situations may contribute to increases in the precision with which students encode fractional relations in everyday contexts, both as spatial and numerical relations.