Metacognitive monitoring, recognizing when one is accurate or not, is important because judgments of one's performance or knowledge often relate to control decisions, such as help seeking. Unfortunately, children and adults struggle to accurately monitor their performance during number-magnitude estimation. People's accuracy in estimating number magnitudes is related to math achievement and health risk comprehension. Thus, poor monitoring of number-magnitude estimation performance could pose problems when completing math tasks or making health decisions. Here, we evaluated why monitoring accuracy was so poor during number-line estimation, whether it was greater in the presence of a cue that was predictive of performance or when accounting for spatial skills, and the relation between monitoring judgments and control. Monitoring accuracy was greater in a condition in which familiarity, a cue adults commonly rely on to monitor their performance in this task, was predictive of estimation accuracy, compared to a condition in which familiarity was misleading. Although indices of monitoring accuracy did not improve when accounting for spatial skills, reducing variability by dichotomizing estimation performance into accurate or not improved monitoring accuracy metrics. Accurate monitoring was important because adults were more likely to ask for help when they were less confident in their estimate. Taken together, our data on monitoring accuracy suggested that (a) using cues predictive of accuracy is important for monitoring in number-line estimation, (b) adults are poor at detecting small differences in their performance, and (c) prior estimates of monitoring accuracy in number-line estimation may underestimate people's true monitoring ability.