The purpose of this study was to define the effectiveness of a smartphone-based dosage calculation training app on nursing students’ learning achievement, metacognition, and flow based on prior knowledge. The study used a quasi-experimental design with a pre- and post-test. Participants were 157 nursing students from 3 universities with baccalaureate programs in South Korea. After recruiting the experimental and control groups, they were also categorized by prior knowledge of drug dosage calculation ability into above and below-mean clusters. The experimental groups were provided with the smartphone-based app. In the above-mean cluster, changes in total learning achievement (Z=3.16, p=.002), drop rate calculation (Z=2.76, p=.006), metacognition (Z=2.50, p=.012), and flow score (Z=2.42, p=.016) in the experimental group were significantly higher than those in the control group. A smartphone-based calculation training app improves calculation achievement, metacognition, and flow among students with higher prior knowledge. For learners with lower prior knowledge, additional instructional design or implementation strategies may be needed.