Self-evaluations play an important role in various fields of study, specifically in research on metacognition and self-concept. Although the assumption that self-evaluations as known from metacognitive monitoring and academic self-concept are related has received wide agreement, the nature of such a relationship has only rarely been investigated. In the current study, the individual-differences approach that has occasionally addressed this association is discussed and extended twofold. For one, a novel way to compare metacognition and self-concept is presented by computing a self-concept bias-analogous to metacognition research. For another, the study targeted a younger population, namely first-grade children. In line with previous studies, the results confirmed a weak relation between metacognitive monitoring and academic self-concept when relating the two constructs at the absolute level of confidence. However, relating the constructs by means of the respective biases revealed a more substantial association. Thus, while previous studies have assumed the common thread between metacognition and self-concept to be best explained by a general confidence trait, the present study suggests the accuracy of self-evaluations to be at stake instead. Hence, by introducing a method to quantify a bias in self-concept, the current study proposes a new and promising way to compare and relate the constructs of metacognition and self-concept.