Through a synthesis of test publisher norms and national longitudinal datasets, this study provides new national norms of academic growth in K-12 reading and math that can be used to reinterpret conventional effect sizes in time units. We propose d' a time-indexed effect size metric to estimate how long it would take for an "untreated" control group to reach the treatment group outcome in terms familiar to educators--years/months of schooling. It serves as a supplement to conventional effect size metrics such as Cohen's d by taking into account different amounts of time needed for learning at different age or grade levels. Through applications to Project STAR small class effects and NAEP [National Assessment of Educational Progress] racial achievement gaps, we demonstrate how to interpret and use d'. It is expected to provide a more developmentally appropriate context for interpreting the size of an effect, a step toward bridging the gap between educational research and practice. Descriptions of test data used for K-12 reading and math achievement growth norms are appended. (Contains 5 notes, 5 figures, and 4 tables.)