Many countries include mental calculation within their curriculum, as the literature shows benefits related to it. However, evidence mainly focuses on the effects of teaching mental calculation on computational fluency. Therefore, the authors aimed to assess the effects of teaching mental calculation on double-digit computation, number line estimation, and computational fluency. Fifty second-grade students from a Uruguayan private school were randomly assigned to a control or treatment condition (n = 25 each). The treatment group participated in 15 sessions of mental calculation intervention, while the control group received "teaching as usual." Assessment took place before and after the intervention. The authors found no significant impact of the treatment on the mathematical outcome variables. The results are explained in the context of the available literature and the methodological decisions taken. From a perspective of evidence-based teaching practices, the present study contributes to a better understanding of effective instructional principles.