The in-plane deformability of diaphragms plays a key role in the seismic response of structures, particularly affecting the distribution of horizontal forces and the overall behaviour of the lateral load-resisting system. This study proposes a new theoretical approach to evaluate diaphragm deformability, based on a simplified yet rigorous mechanical model. The proposed method allows for a direct estimation of the diaphragm stiffness, accounting for the geometric configuration and mechanical properties of its structural components. The theoretical predictions have been compared with results obtained from a simplified numerical model developed in SAP2000, showing good agreement and confirming the reliability of the approach for preliminary design assessments or rapid evaluation purposes. The proposed methodology offers valuable insights, especially into the seismic design and assessment of both new and existing buildings.