This work focuses on the development for the electromagnetic optical modellingof a commercial III-nitride vertical LED/nano-LED based on the finite-differencetime-domain (FDTD) method. The material properties, boundary conditions andsource emission are thoroughly investigated. To achieve a reliable model, results obtained from FDTD using a near-to-far-field transform are compared with those obtained using experimental angular photoluminescence (PL). LED parameters are extracted to quantify the performance of the devices and predict the emission pattern of practically infeasible nanorod arrays on vertical LED structures. It is demonstrated the impact of the shape and dimensions of the nanorods has a significant impact on the light extraction efficiency, however further optimisation is required to significantly increase directionality.