Individuals' perceptions of their own level of cognitive ability are expressed through self-estimates. They play an important role in a person's self-concept because they facilitate an understanding of how one's own abilities relate to those of others. People evaluate their own and other persons' abilities all the time, but self-estimates are also used in formal settings, such as, for instance, career counseling. We examine the relationship between self-estimated and psychometrically measured cognitive ability by conducting a random-effects, multilevel meta-analysis including a total of 154 effect sizes reported in 41 published studies. Moderator variables are specified in a mixed-effects model both at the level of the individual effect size and at the study level. The overall relationship is estimated at r = .33. There is significant heterogeneity at both levels (i.e., the true effect sizes vary within and between studies), and the results of the moderator analysis show that the validity of self-estimates is especially enhanced when relative scales with clearly specified comparison groups are used and when numerical ability is assessed rather than general cognitive ability. The assessment of less frequently considered dimensions of cognitive ability (e.g., reasoning speed) significantly decreases the magnitude of the relationship. From a theoretical perspective, Festinger's (1954) theory of social comparison and Lecky's (1945) theory of self-consistency receive empirical support. For practitioners, the assessment of self-estimates appears to provide diagnostic information about a person's self-concept that goes beyond a simple “test-and-tell” approach. This information is potentially relevant for career counselors, personnel recruiters, and teachers.