Understanding fraction magnitudes is especially important in daily life, but fraction reasoning is quite difficult. To accurately reason about fraction magnitudes, adults need to monitor what they know and what they do not know. However, little is known about which cues adults use to monitor fraction performance. Across two studies, we examined adults' trial-by-trial fraction estimates, confidence judgments, and ratings of fraction familiarity. Adults were more confident when their estimates were more precise as well as when estimating fractions they rated as more familiar. However, adults judged their confidence in estimating fraction magnitudes, in part, based on their familiarity with each fraction. The role familiarity cues play in judgments of confidence with fractions suggests that people may be less likely to check for errors when reasoning about highly-familiar fractions.