Magnitude understanding and metacognition are important in life and for academic achievement. In two studies, we examined children's and adults' metacognitive awareness of their wholenumber and fraction magnitude estimates. There were few differences between grades or numerical ranges in third through fifth graders' (8-12-year-olds') and adults' (18-59-year-olds') item-by-item confidence and familiarity judgments, even when there were differences in estimation precision. A brief experience with unfamiliar fractions did not lead to greater familiarity or confidence, likely because of participants' extensive prior experiences with numbers. Monitoring accuracy was poor across grades and number types, suggesting it may be limited in number-line estimation. Additionally, participants relied on their familiarity to monitor their performance more during fraction than whole-number estimation, likely because people thought many large whole numbers were equally unfamiliar. Our data suggest people use different cues to monitor their fraction and whole-number magnitude estimation performance, yet monitoring accuracy is similar across grades and number types.