Learners have difficulties applying mathematics in the real world. This design–action research explored the heuristics of the Realistic Mathematics Education (RME) approach for learning calculus. Ten in-service student-teachers completed a questionnaire with open-ended and closed-ended items on their experiences in applying Calculus in both decontextualised and contextualised settings. They also completed two assessment worksheets. The data were analysed for themes, and statistical paired t-test for comparison. The findings indicate that the teachers had positive learning experiences, even though there was no statistically significant difference in their performance. This suggests that the RME approach can be effectively used in the mathematics classroom to create an interactive learning environment, and to facilitate students in solving real-world problems.