One of the strongest predictors of future academic achievement is the early and informal math skills children begin their school studies. Because of this, it is essential to have proper tools for measuring the development of mathematical thinking at an early age to be able to intervene in a timelier, more effective way. The purpose of this research is to calibrate the items of informal mathematics from the test of early mathematics ability–third edition (TEMA-3) by applying the Rasch model. A total of 148 Peruvian preschool children (ranging in age from five to six years) participated in the study. The results show good psychometric properties of the informal mathematics dimension of the instrument, which indicates a good fit of the student sample, the items to the proposed model and a tendency toward unbiased items. We further determined that the items analyzed exhibit a consistent internal structure at the theoretical level.