Young children share fairly and expect others to do the same. Yet little is known about the underlying cognitive mechanisms that support fairness. Across two experiments, we investigated whether children’s numerical competencies are linked with their sharing behavior. Preschoolers (aged 2.5-5.5) participated in either third-party (Experiment 1) or first-party (Experiment 2) resource allocation tasks. Children’s numerical competence was then assessed using the Give-N-Task (Sarnecka & Carey, 2008; Wynn, 1990). Numerical competence – specifically knowledge of the cardinal principle explained age-related changes in fair sharing in both the third- and first-party contexts. These results suggest that an understanding of the cardinal principle serves as an important mechanism for fair sharing behavior.