The influence of digital media on personal and social well-being is a question of immense public and academic interest. Scholars in this domain often use retrospective self-report measures of the quantity or duration of media use as a proxy for more objective measures, but the validity of these self-report measures remains unclear. Recent advancements in log-based data collection techniques have produced a growing collection of studies indexing both self-reported media use and device-logged measurements. Herein, we report a meta-analysis of this body of research. Based on 106effect sizes, we found that self-reported media usewas only moderately correlated with device-logged measurements, and that these self-report measures were rarely an accurate reflection of logged media use. These results demonstrate thatself-reported measures of the quantity or duration of media use are not a valid index of the amount of time people actually spend using media. These findings have serious implications for the study of media use and well-being, suggesting that cautiousness is warranted in drawing conclusions regarding media effects from studies relying solely on self-reported measures of media use.