Prior research has found gender differences in spatial tasks in which men perform better, and are more confident, than women. Do gender differences also occur in people’s confidence as they perform number-line estimation, a common spatial-numeric task predictive of math achievement? To investigate this question, we analyzed outcomes from six studies (N = 758 girls/women and boys/men with over 20,000 observations; grades 1–5 and adults) that involved a similar method: Participants estimated where a provided number (e.g., ¾, 37) was located on a bounded number line (e.g., 0–1; 0–100), then judged their confidence in that estimate. Boys/men were more precise (g = .52) and more confident (g = .30) in their estimates than were girls/women. Linear mixed model analyses of the trial-level data revealed that girls’/women’s estimates had about 31% more error than did boys’/men’s estimates, and even when controlling for precision, girls/women were about 7% less confident in their estimates than were boys/men. These outcomes should encourage researchers to consider gender differences for studies on math cognition and provide pathways for future research to address potential mechanisms underlying the present gender gaps.