Optimal repeater insertion is important in VLSI(very large scale integration) interconnects to reduce propagation delay and power dissipation. We propose a particle swarm optimisation (PSO)-based optimal repeater number for different lengths of carbon nanotube (CNT) interconnects at 20 nm and 14 nm technology nodes. First, numerical equations for optimal repeaters are modelled. Secondly, a PSO-based algorithm is developed and various input parameters are used to train the PSO. Optimal number of repeaters and the propagation delay for CNT interconnects for lengths ranging from 500 to 2000 µm are found out. Results are compared for both numerical and PSO methods, and it is found that they are within 2% deviation. Although earlier work shows similar findings, we have used CNTFET(carbon nanotube field effect transistor)-based inverters as repeaters in CNT interconnects for the first time. We have carried out the analysis for two values of repeater size, i.e. h = 50 and 75. © 2021 Informa UK Limited, trading as Taylor & Francis Group.