This study was conducted to assess the efficacy of a design heuristic to assist teachers in devising flipped classroom (FC) lesson plans based on the 5E instructional model. Data comprised lesson plans collected from 121 middle mathematics teachers in Saudi Arabia who used the design heuristic during a four week online professional development course. A document analysis was performed to examine how the teachers utilised the design heuristic. This led to the identification of three principal categories: Pre out-of-class phases to stimulate students, in-class phases comprising student-centred learning activities, and post out-of-class phases to consolidate what was learnt. The lesson plans were then analysed using the 5E model scoring instrument. The results revealed that teachers planned to use Pre out-of-class phases for engagement, post out-of-class phases for consolidation, and in-class phases principally for carrying out student-centred activities. Overall, the analysis revealed that middle mathematics teachers had the ability to design FC lesson plans that were largely aligned with all five phases of the 5E instructional model. Based on the findings, the researcher recommends the use of flipped classroom approaches aligned with the 5E instructional model in different subjects and at various stages of education. This study also concludes with recommendations regarding future research in this area.