To facilitate a more definitive understanding of the usefulness of formative assessment, this meta-analysis examined the impact of formative assessment on student academic achievement in the K-12 classroom. The study analyzed 258 effect sizes from 118 primary studies published around the world. By applying a broad lens for defining and understanding formative assessment, the study confirmed the usefulness of formative assessment. The overall effect size was 0.25 (Hedges' g) for all studies included, and 0.22 for studies conducted within the US. The effect size was similar across different types of formative assessments (e.g., self-assessment vs peer assessment) and content domains (e.g., English language, mathematics, science, etc.). The mean effect size was significantly larger for studies conducted outside North America and Western Europe, for formative assessments for high school students, and for studies using the quasi-experimental design. No publication bias was detected for studies conducted in North America and Western Europe.