The involvement of working memory (WM) was examined in two types of mental calculation tasks: exact and approximate. Specifically, children attending Grades 3 and 4 of primary school were involved in three experiments that examined the role of verbal and visuospatial WM in solving addition problems presented in vertical or horizontal format. For Experiment 1, the children were required to solve addition problems with carrying. For Experiment 2, they were required to solve addition problems without carrying. Then, for Experiment 3, the children needed to solve approximate problems with and without carrying. Results confirmed that different WM components are involved in solving mental addition problems. In Experiment 1, horizontally presented addition problems were more impaired than vertically presented ones, according to a verbal WM load; conversely, vertically presented addition problems were more affected by a visuospatial WM load, especially when the children were required to perform approximate calculations. In Experiment 2, this pattern emerged in neither exact nor approximate calculations. Finally, in Experiment 3, the specific involvement of WM components was observed only in problems with carrying. Overall, these results reveal that both approximate calculation and carrying procedures demand particularly high WM resources that vary according to the task’s constraints.