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.