The development of flexible arithmetic skills has increasingly come into focus in the subject-specific didactic discussions of recent years. At the same time, studies such as TIMSS show that many children still leave elementary school without basic arithmetic skills. This raises the question of the extent to which children who initially struggle with learning to calculate in the first grade can transition away from counting within regular instruction and further develop flexible arithmetic skills. This question is at the center of a qualitative learning process study, in which activities to train number sense were regularly conducted in five first-grade classes throughout the entire school year. The results of the study suggest that training in number sense supports both the transition from counting-based calculation and the development of flexible arithmetic skills.